I have a 7290 from EBAY with no SIM. I inserted my AT&T 3G SIM and the thing works, but will not connect to the data network. Is this a common problem? Will the 3G SIM work in the 7290?

The SIM will work fine as long as it's properly provisioned with a BlackBerry Data Plan.
There's really no such thing as a 3G SIM. It's just a SIM card, with some marketing stuff printed on it.

Actually there is a difference between 3G-capable cards and others. Cards made for 3G phones have more memory on the chip. A typical 2G card has 32k of memory, while a 3G card has 64k or 128k.

They are made to be backwards-compatible with older phones, but some phones are finicky.

For what it's worth, my at&t 3G card works fine in a 7290.

The difference in memory capacity doesn't impact its ability to operate on a 3G capable network.
Older cards with less memory will still work fine on 3G networks.

Not necessarily.

OK, you win.