I got this one:
http://trendnet.com/products/proddet...=280_TU2-ET100

It works "out of the box" (note my N900 setup in my signature, though). However:

a) on one network, I got the same "bogus packet" messages with udhcpc.

b) on another network, it works fine at first, but it seems that after a while the web browsers (microb and Opera) "forget" that they are connected, and never download the requested page; ping and wget from XTerminal still work. Also, turning on the online accounts in My Availability causes the N900 to connect to the default network (3G); this is even before the web browser problem sets in.

c) power consumption: the dongle runs pretty warm (though it can still be held) and will drain a full N900 battery in roughly an hour. The declared consumption is 178 mA.