Partners and customers deploying Office Communications Server 2007 with enterprise voice still require a media gateway for a supported configuration. The definitive place to determine supported vendors, models and the qualification level (direct sip via gateway, direct sip, dual forking and dual forking with remote call control) you need to check the Open Interoperability Program site: http://technet.microsoft.com/ucoip
I wanted to share a common configuration that we are seeing our partners asking to deploy that is not part of the certification process and thus not supported - dual usage of a Cisco router (other vendors use term gateway thus my referencing both names).
Below is a diagram we used amongst ourselves when first asking about this being supported (thanks Jochen). You can see that there is only 1 gateway servicing both a CCM and OCS deployment. If you refer to the diagrams in the planning workbook (still lacking a post discussing the doc), the diagrams show what we sometimes refer to as back to back gateways.
At this time this is not supported simply because it was not a part of the certification process and thus nobody knows what issues may or may not exist. If this topology is of interest to you then please communicate it to your account manager (partner or customer).