Well after doing some tests I don't think the LAN speed is a factor at all; you're correct in that the CPU does get utilized when doing USB transfers, but the controller seems to operate in a different way than when doing network transfers as the speeds that I get when testing over the network are not the same as a direct USB connection. In the end using MaemoFTP I got sustained transfer rate of 700 KB/s (approx 5Mbps) across either Wireless or USB wired network; the CPU at this point maxes out and cannot push any more data across. I haven't tried with the browser, but I imagine if you do a similiar test you'll probably end up with the same sort of results (probably a little higher but around the same sort of range) in that the tablet will go quite quickly, but that's quickly in terms of Internet speeds, not actual LAN speeds.
I know there's been a lot of work on mouse support (there was talk of an app that would poll for a mouse and enable it if found/etc), I just haven't kept up with that thread. None the less, just don't expect it to work "out of the box" under Maemo, that's all! Note I say Maemo, not "N800"... if you're running KDE, you're not running Maemo...
And when I say possible for the serial adapters, I mean I can get one or two of them for a reasonable price from the same supplier as my other equipment, so it's commerically viable even if there isn't a lot of demand. Some of the other things are not in the same boat, so I need to make sure there is a demand before I purchase them in quantity. One more thing... I may have a lead on a VERY COOL USB cable/device, I'll give you more info when I hear back from my suppliers about availability. Thanks, -Rob