So it turns out my problem above is that stock Android PAN DHCP is just super unreliable, that's all(unless a Cyanogen update just fixed a problem). I give it a try AND wait for 3.5G to connect; it turns out that I eventually get a connection as long as I click "Internet Access" in the settings, verify that 3.5G is up on the N900, then check for actual access in a browser; rinse and repeat sometimes two or three tries until success.