EDIT 10/27/2009: Please see the updated statement on this subject here.
Please be advised that Single Label Domain (SLD) support in the next version of Exchange is under review.
In February 2008 we announced that Exchange 2007 would be supported on an SLD. At the same time we announced that the next version of Exchange would not be supported on an SLD.
This policy is now under review and it may be that the next version of Exchange will be supported on an SLD. The Exchange product team's view continues to be that SLDs are not a recommended configuration, and that customers should fully evaluate how remaining on an SLD impacts their environment. Anyone who is currently planning on migrating their SLD to a Fully Qualified Domain Name (FQDN) only because of the February 2008 announcement should also take into consideration that the policy is being re-evaluated. We expect to make the decision public in the next few months.