Won't work. They are just using dpkg to package and distribute their apps, they don't even seem to be using much of dpkg's features: http://www.saurik.com/id/7

Regarding the "removed support for installing .deb files in Application Manager" - you can always install .debs using dpkg from the command line (start X-Terminal, "sudo gainroot" and then "dpkg -i /path/to/debfile.deb"). But even if you managed to extract or install the iPhone .deb on your Maemo device, the result will be as useful as having a Linux binary on your Windows machine.

Your best bet for getting iPhone apps to Maemo would probably be to have the source code for the iPhone app, take the backend code, and write a new UI for Maemo on top of that. Given that most iPhone apps probably make heavy use of some Mac OS-specific frameworks and libraries, this is going to be even harder, as you would have to replace calls to these libraries with something else (and at that point, re-writing from scratch is usually the better alternative..).

Porting existing applications from Desktop Linux is probably easier. For things like biteSMS (sending SMS through the web AFAIU), the major functionality happens on the server-side, so you would only have to write a Maemo client for that service. I've even written a WebSMS client app a long time ago that sends SMS through the internet connection (although it's limited to just one cellular operator in Austria), so things like that are surely possible