Not many do. There are quite a few things I build myself if I really really want to use it. A lot of the time it isn't worth it.
I would totally be using MongoDB for some projects (it fits the bill -perfectly-), but my resources for those are usually limited to Solaris on SPARC.
I do have an x86 desktop with Linux as my main PC at work, so I don't miss out on all the fun completely.
However, I do mostly sysadmin stuff, so a lot of the things I use come in the form of scripts, so it's often cross-platform.
Though I do come across some install scripts that just blatantly assume that everything is running Bash on Linux. Those are fun.
I have very little expectation that the MongoDB team will ever push a non x86 version. They do a lot of optimisations deep inside that rely on architecture specific things. But one can hope :)
Lots of big companies, mostly infrastructure like telecom or tv, use Solaris still. Unless the project aims to support those uses, it usually isn't worth their time.
Of the open source projects you follow, how many still do builds for Solaris? How often do you have to build them yourself?