Trying to make a list..

1) - Wrong WEP key, the error symptoms aren't obvious so may look like network problem
2) - MAC filtering, the network isn't truly open
3) - Bluetooth interference, needs channel change on AP
4) - Access points interference, i.e. some other network nearby, needs channel change on AP
5) - Configuration problem inside roaming network, e.g. one AP has incorrect ESSID - works occasionally but isn't stable. AP must be reconfigured
6) - Occasional problem inside the AP, this is the nature of cheap APs - need to restart AP (yank the power cable for a few seconds)
7) - Some early 802.11g routers are slightly outside spec, they work with some equipment but not with others. If it works ok in 802.11b mode this is it. Be wary of "108Mbit" APs.
8) - Since the N800 is so sensitive it may see an AP but the AP doesn't see the N800. Move closer to get a stronger signal.
9) - Some unrecognized/non-understood 770/N800 problem.

Of the above I've experienced 5 and 6, others have reported the rest with the possible exception of 7 - I think so, but I'm not certain. However, I've seen this with other (non-tablet) equipment. 9 is speculative at this point, but cannot be dismissed yet.

Did I forget anything?