read this topic:

http://www.summitpost.org/phpBB2/viewtopic.php?t=42556

The route up to the Southern Icefields via valle Río Electrico can be quite bad to your feet if its newly snowed over - many big blocks and rocks make you stumble...Also beware that IF you should cross over to Chilean Territories the border crossing is pre-cleared (your guide/agency should take care of this).
If you are not tied to Chalten area, there are other great ways of access to either the Northern or the Southern Patagonian Icefields, like the Jorge Montt glacier and Steffen glacier. Also, there are some stunning spires on the Northern end of Southern Patagonian Ice Cape which have never seen a human on top...
Winter time might be best to come (August/September) since conditions are often much more stable then.
Cheers,
Olaf