Yes, if they don't use any of the Hildon stuff. GTK+ dialogs also look a bit strange (no decorations) in the current MeeGo firmware image for the N900. The apps must not access Maemo-specific stuff through D-Bus services that are not available on MeeGo. Most Maemo 5 GUI apps use at least parts of Hildon (if they don't use Qt), though (things like kinetic scrolling, large buttons, the app menu, etc..) - so porting these will most likely require some changes.

That's probably the smallest issue You could either have dpkg installed in MeeGo as well (and don't forget that Harmattan aka "the first MeeGo-ish OS that comes out with the next Nokia phone" is still using DEB), and there are tools like alien which can convert between RPM and DEB. Ignoring some technical details (like scripts and dependency information amongst other metadata), packages can be though of as archive files that simply contain all files as they are installed in the file system - if the dependencies are satisfied and the binaries are compiled for the correct architecture, there's no no problem with taking the contents of a .deb package and setting it up inside an RPM-based system (that is, if you don't have the source available - otherwise you could just create a Spec File and build a "native" RPM, which is less of an issue than changing the source code and rewriting code to remove the Maemo 5 dependencies and replace them with their MeeGo equivalents).