Yeah, definitely rotate this to have the bottom of the LEGO brick down, sorry that wasn't clear!

I just made a couple of these. The first one was printed with the default orientation from the STL file, on a Replicator 2 in PLA, with default Medium quality settings in MakerWare... no supports, no raft. This first piece works with real LEGO pieces to a certain extent, better than any other bricks I've tried to print from other models but still not perfect. There was some drooping on all of the overhang so the fit isn't 100% but at least they do snap on.

The second piece was printed after rotating it to be upright to avoid too much overhang, this worked a lot better. We ended up needing to use a raft for this one. Same default medium quality in MakerWare, this brick also works with real LEGO bricks, a little bit better than the first in fact, the only drooping is on the inner ceiling of the brick.

I'll try to upload some photos of both soon, they were done in white filament from SainSmart.