The 3 major differences are 40 party conference calling,support for the 1100 series IP sets and, active USB ports onthe front of the system. Other than that there is not much difference.
The next BCM release I am hearing is going to 4.5 with the ability to support 300 sets IP and digital and 5 T-1's.
The release is set for sometime around June 2008.
Knowing nortel's projected relerase dates, that will be sometime around 2010.
Also along with this release I am hearing there will be a Base function tray upgrade to bring older BCM's up to 4.5.