I am no expert, but the SPEC file at http://github.com/mongodb/mongo/blob/master/rpm/mongo.spec does not work for me, and i'd thought i'd post the two minor changes that are needed to get it to work (at least for me).
Firstly, file downloaded for the SOURCES should be renamed to mongodb-src-r1.7.2.tar.gz as this is what you download on the website, i.e.
Then, this file extracts differently too, so this should be updated:
+%setup -n mongodb-src-r1.7.2
There is a bug in that the man page of mongosniff is included, but not the binary:
The RPM will then build fine.