After spending a few weeks working with Cisco in trying to figure out why new 1252 APs weren't getting a DHCP address from our Juniper SSG5 firewall (on different VLANs for each SSID), I finally got to a Cisco tech who knew the answer, so am posting it here to perhaps help someone else.
Using MBSSID, according to the first tech, only one can be in open authentication mode (e.g. no WEP, WPA, etc.) You can have multiple SSIDs that utilize the same method of authentication, just not more than one of them open.
Using MBSSID, on some wireless NICs that are apparently somehow sensitive to it, only one SSID can be in guest/broadcast mode. This ultimately was the solution to our connectivity issue.
It would be good to have documented what those NICs are that experience this problem.