The BCM450 is currently on Nortel's Plan of Record (PoR) for delivery in November of this year (in North America). Anticipate much improved scalability in both users and trunks, as well as full appication parity with R3 of the BCM50.
An upgrade path for all 400s is also planned which simply involves replacing the Base Function Tray. Should be a relatively simple upgrade.
This is all I can post for now, folks, as any additional information would be under NDA. If you need more details, talk to your direct Nortel representative or authorized reseller/channel!