I did a search on google and it comes up with the usual 'ahem' places to download various tutes on the subject.

But I just had a look in my character animation book - and there it was!!

The book I have is by Jae Jin Choi, its most likely PDf'd somewhere so check it out on the web. The section for pushing a cube is about halfway. I'll have a look on the dvd and see if there is a scenefile and I'll mail it to you. he uses simple orient and point constraints...

Cheers
Jay