While not having a DEB packaging GUI is one problem, another problem is that Nokia somehow "broke" dpkg due to the current BusyBox implementation "breaking" tar. That's one reason why none of the old themes or themes created with the theme creator program will work on v4 of the firmware (the Skype-enabled firmware).
It's been noted as a bug and one of their engineers has fixed it, but it's not user-upgradable yet.

I would also love some sort of GUI for creating DEB packages, even if it's only for the N770/N800 as I've also ported over a program or two and right now I'm left with using .tgz files installed as root from the root directory.
And reading over the "official" Debian packaging guide makes my eyes bleed.
There is an easier way - done from the command line of course - but again, I think this method is broken due to the current status of tar in the BusyBox build that's present in the v4 firmware update.