Try this:
=RIGHT(TRIM(A2),LEN(TRIM(A2))-FIND(" ",TRIM(A2)))
I was able to copy/paste the formula and it worked fine.
Here is a list of Excel text functions (which worked in May 2011, and but is subject to being broken the next time Microsoft changes their website). :-(
You can use a multiple-stage-nested IF() functions to handle middle names or initials, titles, etc. if you expect them. Excel formulas do not support looping, so there are some limits to what you can do.