You will have no problem with bringing models from earlier versions of maya into 2011. When opening a file from 2011 into a previous version goto file> open file and then option box next to this. You can now check ignore version and it will open fine. You will sometimes get error messages although this tends to happen if things are not clean in your outliner and if rigging is involved. A lot of the weighting and rigging in maya 2011 had an overhaul.

Hope this helps