1/ in 1999 all of our developers had Win-based developer towers with exchangeable HDs (not hot-swappable, though). I could turn of the office PC, slide out the 5.25" HD, and put it into developer PCs at home or other office locations. two drawbacks: some software registered by using CPU serial numbers or mainboards or something (I'm not a techie), so some software had to re-register (like Photoshop does now when you change the hardware).

2/ T-Systems (like many others as well) has "managed desktop services" since 2003, which in fact is a virtual desktop environment similar to SUN's Sunrays - you can use a low-power-box, an all-in-one LCD screen, or software running on your computer, all works with only few problems (HD video is one of them; a local flash, java, silverlight rendering engine on-chip solved that problem, too).

3/ OnLive, best known for its gravitation it currently creates for "Game Streaming", is planning to release a business suite - if latency and frame rates are okay for gaming, it should be OK for browsing or word processing. Of course difficult if you're not always online ;)

4/ HTC concept phones do exactly what you describe - problem is business mechanics and balance of computational power: should you buy a cheap phone and a fast laptop or a fast multicore phone and a dumb screen, where only one core is active when it's a phone, and 16 cores are active when it's a PC....

5/ Several concepts exists for a "Facebook OS" mobile phone. Together with Facebook Apps, you have a limited ubiquitous functionality... of course none of that has the potential of an all-purpose PC, and I guess it will take a while until the "Photoshop Facebook Application" will come out, and netbook vendors won't sit on the sidelines, either...

6/ License models and software activation lags behind the "OS in a cloud". I didn't find any good startup or incumbent software maker yet that actually (not per marketing) addresses this problem.

7/ I know two under-the-radar startups in the valley that create BIOS / Firmware that can boot over Ethernet or Wifi...

8/ I tried to map a web-based storage drive once and install Photoshop into that web drive. Besides the already mentioned license activation problem, on a win PC the installer throws files and stuff everywhere across the HD, personal profile folders, windows systems dir, common files, ... a self-contained installer would be necessary (hello Adobe Air). Second problem was that most online storage programs "optimize" the upstream updates by caching or doing something else that locks files... e.g. updating ZIP files or creating them on-the-fly was a problem in earlier Dropbox versions (no idea if that's still the case).