I'd love that and quite frankly the potential for this option is an important reason why I want to get a Neo900.
I consider Fremantle more or less a fallback in case Debian turns out not to work as I hope.

Unfortunately I don't have the skills to port Hildon on my own, so basically what I'll be trying to do is get Debian/armhf running on the Neo900 with some UI already in Debian that works on the small screen.
So far the best I've come up with (in a 800x480px VirtualBox window) is Enlightenment.
At some point I realized that what I was doing was pretty much the same Jeff Hoogland has already done with his Bodhi distribution, so I started to look for inspiration and even to copy parts from Bodhi to my Debian vbox. (@Jeff: in case you read this, thanks a lot!)
One problem I encountered is the stability of e17. Of course this will have to be evaluated on the target plattform, but I'm kind of sceptic if it will be stable enough for my needs.
As a fallback I also tried LXDE, but if you want something less cumbersome that the Easy Debian UI variants traditional desktops just don't work well on a 3.5" touch screen.

In case you feel up to the task of porting Hildon to Debian I'd be happy to assist, but I have no programming skills to speak of and my packaging skills are dubious at best.
I tried to run matchbox in Debian and I tried the hildon package that was in Squeeze. None of them seemed to be promising.