How secure is 802.1x? Is it as easy to bypass as port security using mac addresses? Can you spoof the SID of a computer and join to the network or is there further verification that is done through the RADIUS server outside of just the machine or user SID? I am about to dig in and research this further but I figured this would be a great place to start.

I think the "how secure is 802.1x" depends on how much 802.1x you're willing to implement, it's a bit of a broad standard from what I know.

You can require computer certificates, only available over an initial wired connection (if you're talking wireless) and a valid user cert once you've logged on before being dropped into the right valan etc all do-able with a 2k3 AD. Plus side of requiring both certs is you can revoke either

Yeah getting the initial comp cert could be a pain if you've got a huge amount of laptops to secure, but it only needs to be done once. I've seen it done with a couple of hundred machines in very little time.

This is all 802.1x for wireless connections, which seems fairly nails. - wired I've had no experience with.