This isn't going to be the answer you waht to here, but there is an R package for it:

http://www.inside-r.org/packages/cran/insol/docs/insolation

So if you can wrap the point data from your DEM into a simple for loop (lat/long/elev) you should then be able to output a file (after some judicious data choices) with daily, monthly or annual insolation for each point in the DEM, although this would obviously be affected by things like weather, but there's daily models for that as well, or at the very least, weather station data.

To wrap an R script in matlab you just use the call:

system('R CMD scriptfile.R')

Thanks Simon - it's unclear whether that routine takes topographic shading into account, which of course was not part of my original request ;)