Yep, compileSdkVersion just sets which version of the API SDK the code is compiled with.  There isn't anything in the build or source code now (got taken out) specifying a supported API range (which is a risk).  OSMDroid supports all the way back to 4, but based on market usage MapBox could probably get away with Version 2.2 (aka API 8, aka Froyo) as baseline although Gingerbread (API 10, Version 2.3.x) is probably more realistic.

I've reopend #53 to put this range back into the build.  It'll also help during development so that API levels on methods are enforced by the IDE.