I've heard of interest in subbing Drama CDs before, but it almost never happens. I'm afraid that I only know of two Drama CDs that were ever subtitled. It is a terrible shame that there aren't more. For a little while some people were talking of subbing the Mai-HiME Drama CDs and while a member posted scripts on our site, nobody ever got serious about subbing them.
In case you're curious, there are two fansubbed drama CDs listed on AnimeSuki.
Evangelion Audio Drama
Twelve Kingdoms Drama CD