A considerable number of owners of the new 15” and 17” 2011 MacBook Pro’s have reported both on the Apple Discussion board and also on the MacRumors forum that their new machines are locking up and freezing when under an extensive load. For example this post by 'brandonlive02' is representative of what many are complaining about;
I have a 15" 2011 MacBook Pro with a 128 SSD. It has repeatedly frozen in the week I've had it - often at random points while using StarCraft II and also while trying to access Time Machine. When it freezes, sound continues and I can still move the cursor (which is sometimes the spinning ball and sometimes the regular cursor) but the computer is completely unresponsive -- force quit does not work nor does anything else - have to do a hard reboot.
The issue is believed to be fairly widespread with one user reproducing it on three separate machines and another who reportedly managed to reproduce it on every new MacBook Pro at their local Apple store. This wiki outlines the various methods one can use to re-create the crash if they wish to check if their MacBook has this issue.
No complete fix has been found and the cause is still unknown but the wiki page suggests one fix of using some software to only use the integrated graphics chip of the MacBook Pro. A post in the MacRumors thread suggests that Apple is aware of the issue and that they believe it to be a firmware or driver related problem rather than a hardware issue, but this has yet to be confirmed and Apple have not issued any public statement.