The two can learn a lot from each other.

And X term is something I actually want on my mobile device. Yet, on such a small screen, you can't get a 80x24 terminal. Which ****s up some applications.

Now, regarding software availability.

1) Many are shareware, demo, nagware. There are also problems with unsigned software.
2) There is no central repository or easy way to upgrade applications. There is no APT or App Store of some kind. Geez, what a load of hoompa to get Viewranger _downloaded_. And then its a trial...
3) Some things are really difficult on S60. The hierarchy of the filesystem is a complete mess. Compare to OSX (not Windows please).
4) Some applications are non-standard, some are. For example, there is no free/open OpenOffice.org-like suite. However, there is some very cool and useful software for S60. And it has the momentum of being developed for. E.g. Google Maps is available for S60 (Maemo has Maemo Mapper though). Although iPhone has this momentum as well.

We'll see some nice competition between Nokia N97 and RX-51 the only thing I wish for, is that the OSes evolve further, learn from each other, and get more compatible (eventually a S60_compatibility_layer, Cygwin_for_S60, or toolkit compatibility via Qt would also be win-win)