There are a lot of options, but like the best is to use something like ADMT
to move the users and data.  Check out the multi-forest scenario in the docs
at http://www.microsoft.com/exchange/library for some of the other issues
you need to contend with while in multi-forest mode.

My advice?  Get migrated as quickly as possible and don't let it linger.

Trusts are not necessarily related to email usage but might instead be part
of a bigger migration strategy.

Al