A feature to allow restriction of IM Gateway Username based on email domain e.g something@mycompany.com would be a VERY useful feature, in order to help enforce users not being able to use their personal accounts for business IM's.
The main gateway we deal with is MSN, but being able to do it on any would be even better.
As a side note to myself – probably the best way to implement this would be to allow for an ad-hoc regular expression for matching valid usernames. Probably even abstract the current checks entirely.