I’ve favorably referenced the UC certs from Entrust a few times over on the OCS Forums and I noticed recently that they’ve had a pretty significant price drop. They used to be priced at $1099 for 2 years but are now available in 1, 2, 3, and 4 year versions, all of which have been reduced in price. Definitely worth a look if you are doing an OCS installation or getting ready to renew your certificates.
Microsoft actually worked with the certificate providers when they created the UC Cert program, which includes other providers as well (check the link for the current list). Any time I’m working with a customer on an Edge deployment I use these certificates – they are simple to obtain, well priced, and can be easily reissued if your names change.