Is it possible to do seamless roaming with APs that have different SSIDS? We have multiple access points around and individual departments choose their own SSIDs for these APs. The signals overlap significantly, and all users know the passwords to all of them. I've seen lots of talk about seamless roaming with wifi networks using the same SSID/password - but I failed to find anything on using different APs. Does the 802.11 protocol allow for seamless roaming amongst the same network (all APs have the same gateway)? In other words, can the APs 'hand-over', so to speak, without dropping the connection?

The decision of how and when to roam is completely up to the client. But if the client can't find another AP with the same SSID, it will begin searching for another BSSID that it can connect to. The client will have to negotiate a new DHCP lease. That will certainly not be "seamless."