OK, I ended up having to call as I never heard back via email. It's way more complicated than I thought.

It turns out that the current batch of certificates they have in hand (at least form BXP1 and BXP3 upgrades) are valid thru June 30, 2011. However, if I were to claim an award after June 30, 2010, then they would have the new batch, which are valid thru Dec 31, 2011.

Interestingly, when I asked why they didn't publish this on the website, the response was that they actually can't be sure what the expiration date is - if the June-expiring certificates get used up early, then they start giving out the Dec-expiring ones.

Bottom line, my interpretation (and that's all it is), is that you have at least ONE YEAR to redeem a claimed certificate, although you my get 18 months or perhaps even longer.

EDITED TO ADD: This is for the upgrade certificates only - the rules for travel award certificates are clearly published as noted by BarkinJ upthread.