"Why did you leave your last job?":

A number of really sharp people have been laid off in 2010, often through no real fault of their own, but it is very common for their employer to come up with a "reason", with the result that the employee may still feel that they *are* at fault.  This leaves them feeling a little sensitive about the question "Why did you leave you last job?".  But it is a question that is very likely to be asked as part of an interview, and with no ill intent (your new prospective employer wants to know that they can make you happy enough as an employee that you will not quit at some inconvenient time), so it is worth planning ahead for it.  Practice.

These two links below sound like good advice to me.  I don't recollect that I ever had the nerve to proactively bring up having been part of a mass layoff *before* being asked "why did you leave?", but I like the rest of the suggestions from the first article, and some of the later ones from the second one.

My major advice:
Be brief and professional, but don't sound evasive.  If it worries you to be brief, remember a long response may not really be as important (for you and the interviewer) as spending time understanding/discussing the prospective job.
Avoid saying anything that might be interpreted as back-stabbing former managers or companies, as an interviewing manager may wonder if you would also be critical of him/her.  "Difficult business conditions" is a term I found useful, it was true and sounds like you tried to understand the company's business needs, YMMV.
If you were part of a mass layoff, say so.  It is not so unusual.

http://www.wetfeet.com/Experienced-Hire/Getting-hired/Articles/The-Do%E2%80%99s-and-Don%E2%80%99ts-of-Explaining-Your-Recession-L.aspx

http://corcodilos.com/blog/916/how-to-say-it-why-i-left

Cheers,
Connie