The list of supported partners/hardware will be constantly changing and expanding. For now check out:
http://www.microsoft.com/uc/pdgtrials/default.mspx
Also, check the Microsoft press page for Unified Communications:
http://www.microsoft.com/presspass/presskits/uc/partners.mspx
Here are a few direct links to some of the partners working with Microsoft and OCS 2007:
AudioCodes:
http://www.audiocodes.com/Content.aspx?voip=2823
Quintum:
http://www.quintum.com/microsoft/index.html
Dialogic:
http://www.dialogic.com/microsoftuc/
Jabra:
http://uswww03.gnnetcom.com/jabra/microsoft_microsite/index.html
Polycom:
https://polycom.centercode.com/callout/default.html?callid=CD0F92FEA3614E68B710AD1C6928619C
Here is a very comprehensive list of hardware compatible with Exchange UM:
http://www.microsoft.com/technet/prodtechnol/exchange/telephony-advisor.mspx
Here are the PBXs tested with UM:
http://www.microsoft.com/technet/prodtechnol/exchange/pbx-partners.mspx