The file debian/changelog, both in head and 1.5.1 has a malformed line. The packages will probably not build on any Debian system, but I've only tested Ubuntu Lucid and Hardy.

Fix is simple, change the line:
– Richard Kreuter <richard@10gen.com> Wed, 22 3 May 2010 16:56:28 -0500
To:
– Richard Kreuter <richard@10gen.com> Wed, 3 May 2010 16:56:28 -0500

Here's the log:
$ debuild
parsechangelog/debian: warning: debian/changelog(l5): badly formatted trailer line
LINE: – Richard Kreuter <richard@10gen.com> Wed, 22 3 May 2010 16:56:28 -0500
parsechangelog/debian: warning: debian/changelog(l7): found start of entry where expected more change data or trailer
LINE: mongodb (1.5.0) unstable; urgency=low
parsechangelog/debian: warning: debian/changelog(l7): found eof where expected more change data or trailer
dpkg-buildpackage -rfakeroot -D -us -uc