congrats, you found a bug.
gis.m fails with the error you posted if you start it from an XY location.
Error in startup script: can't read "prj(name)": no such variable
while executing
"string eq "Lat/Lon" "$prj(name)""
(procedure "MapCanvas::get_mapunits" line 30)
invoked from within
"MapCanvas::get_mapunits"
....
that error now fixed in SVN r37257. (it was only in 6.5svn)
but r36386 doesn't solve all problems for Lat/Lon locations, so still
highly broken for them in gis.m 
(both constrained and unconstrained
modes)
Seb wrote:
I was actually using this simple XY location for some intermediate
operations while processing ocean color level 3 binned data. I used
to do a lot of these things using GMT tools, but now the project has
become too large to handle outside a GIS, so am exploring workflows.
see also instructions at
http://grass.osgeo.org/wiki/Global_datasets#MODIS
http://grass.osgeo.org/wiki/Global_datasets#SeaWiFS
Hamish