Do you mean Debian Lenny, mixed with some Squeeze packages? In which case, you are running a mixed system, not Squeeze proper. That is supportable by it's user only because it would be customised to it's core level!
That's not necessarily so. You can still get the 3.5.10-2 packages for squeeze whe you use the debian snapshot mirror from 2010/05/28, 15:55:48 [1]. That was my black friday :-/ - I kept a local copy of that repository till Tim came with TDE 3.5.12 :-)
[1] http://snapshot.debian.org/archive/debian/20100528T155548Z/