I'm actually in China right now with my N800. In tourist areas and cities it's very common for guesthouses and cafes to have wireless internet these days. I carry around a pocket wireless router for those occasions where I can find wired but no wireless internet (fairly rare these days).

I've never tried using my prepaid sim for data, as my understanding is it's fairly expensive, and pointless considering how common wireless networks are becoming here.

As for the other questions about restricted access and tunneling, restrictions are fairly arbitrary. Proxies work, and I've never had problems using an ssh tunnel to the US.

I mostly use my N800 for navigation (bluetooth gps + maemo mapper) while traveling by bicycle. Works brilliantly. The only quirk is that google street maps for China seem to be off by a few hundred meters for the entire country! Satellite maps are spot-on though.