This is just a few .debs I threw up in case they are useful to others. They are not official and not guaranteed to work (let me know if they don't and I'll try and fix them, of course). I have used the official 6.4.2 GRASS source code and DebianGIS git build rules to create them though, so they should not differ from official sources.
keep an eye on
for official backport packages for GRASS on Debian.
MD5 sums can be found in the *.changes file.
H.Bowman 27 Apr 2012