Well, the best "alternative" is probably going to be the Pandora, since it's basically an N900 except a few months early. With the N900 being as far away as it is, I'll almost certainly be getting one (well, assuming they actually do end up being available around November).
Why wouldn't tablets be able to, though? Look at the Beagle. That's what we're looking at performance-wise for the next tablet. It's got more than enough juice to handle the desktop use-case.