I received a PM, a day or so ago, asking about kernels and cameras, with reference to this thread so here is all I know.

I'm no expert: I'm just getting to grips with Linux and the N900 myself but I have managed to resolve the matter of alternative cameras and the Power Kernel to my satisfaction. The following is from my experience and reading various posts on TMO:

1. Camera software for the N900
There are several versions available, including:
(a) Nokia Fremantle PR-1.3 stock camera.
This comes as standard and generally works fine but the software doesn't make use of the camera's potential, hence the reason for other camera software. See 2. (below) regarding Power Kernel.

(b) CSSU UI for stock camera.
This allows better control of the camera, with a very simple UI. This UI replaces the stock one when the CSSU is installed but from what I can tell, it is an augmentation rather than a rewrite, so for HDR etc. other cameras might be better. No additional drivers are required for this camera to work because it uses the stock camera drivers.

(c) FCamera v0.1.7-1.
This camera software has made it through "testing" to "extras".
It is a separate piece of software that extends the capability of the camera to include higher bpp and better jog-tollerance amongst other features. To use it, the FCamera drivers are required. These should install automatically when FCamera (sic) is installed using a repository-based application manager. See 2 (below). for more information.

(d) Bless N900 v0.49-3
This has also made through "testing" it to "extras" and is similar to, but not the same as, FCamera. However, it is proprietary software from Almalence and is in the repository as non-free, i.e. the source is not freely available. This software is good and can be useful, though it can also be slow to load and slow to process images using functions such as multi-pass. FCamera drivers are required to make BlessN900 work, which should install automatically, when using a package manager.

(e) Bless N900 v0.49-4
This is supposed to be exactly the same as v0.49-3, except that it is in the Ovi store hence the version mini-increment. Some say it's buggy but it worked fine for me.
NB i. If the Ovi repo is active in your package manager, the manager will try to update from -3 to -4 but will fail. v0.49-4 may only be installed by downloading it from the Ovi store.
NB ii. FCamera drivers are required to make v0.49-4 work. If they are not packaged with the camera software, the drivers will have to be installed separately.

(f) A Better Camera (a.k.a. ABC) (pay version of BlessN900)
ABC is also made by Almalence and is in the Ovi Store; it does not show up in any repo-based app mgr. Unsure of usage: I haven't tried it. Some say it's buggy. I assume FCamera drivers are also required but not sure if they're bundled with it or have to be installed separately from the "extras" repository. See below for which version. The advantage with ABC over BlessN900 is that it is supposed to load and process photos significantly more quickly than BlessN900.

2. The kernel and the camera
The stock kernel (Nokia fremantle-PR1.3) works "straight out of the box" with the stock camera software, without any alterations to anything (pretty disasterous if it didn't!!). I believe when running the CSSU on the stock kernel, it's just the same: point and shoot. FCamera and all Almalence camera software all need additional drivers. When installing these alternative cameras with a package manager, and using the stock kernel, v0.1.7-1 of the FCamera drivers are installed as dependencies.
To use h-e-n and many other additional features, Power Kernel v0.46 is required I cannot comment on v0.47 because I haven't tried it (or any version other than v0.46). From what I understand, the OTG USB kernel-module is part of v0.46 and is not part of h-e-n itself.
However, Installing Power Kernel v0.46 on a device that has camera software that requires additional drivers, all cameras (even stock camera) will cease to work. I had FCamera and BlessN900 v0.43 installed when I switched from Nokia fremantle kernel PR-1.3 to Power Kernel v0.46 and I got that "operation failed" notification. Having these two already installed when I switched may have been the reason the stock camera stock camera stopped working because it appears Power Kernel v0.46 conflicts with FCamera drivers v0.1.7-1; I don't know for sure.
To get camera funtions to work once Power Kernel v0.46 has been installed (including FCamera and BlessN900), FCamera-driver v0.1.7-2 must be installed.
If Power Kernel v0.46 is already installed before installing an additional camera, ensure FCamera drivers package v0.1.7-2 is also installed or marked as a dependecy so it gets installed automatically.

3. Uninstalling alternative cameras
If Power Kernel v0.46+ is NOT installed, unistalling with a package manager is simple. The drivers should also be automatically removed; if not, do so either with apt-get or with F-App-Man selecting "All packages (ADVANCED)" from the C button. However, I have removed additional cameras but trying to remove FCamera drivers while Power Kernel v0.46 is still installed causes the F-App-Man to flag up the following: "Conflicts: kernel-power (<< 2.6.28-maemo44)". One must assume that, based on this evidence, even the stock camera won't work with recent versions of power kernel unless the FCamera drivers are updated.

Summary:
Stock kernel
- stock camera & CSSU camera - take no action
- FCamera & Almalence cameras - install FCamera drivers v 0.1.7-1
Power kernel v0.46 (OTG USB kernel)
- stock camera & CSSU camera - probably no action needed
- FCamera & Almalence cameras - install FCamera drivers v0.1.7-2
If camera doesn't launch once Power Kernel v0.46 is installed, it's worth installing FCamera drivers v0.1.7-2 to see if these help, even if no additional cameras are installed.

So... hope that was useful.