I got mine about 6 weeks ago or so, and the cert is marked "travel not valid before 12/01/10, not valid after 11/31/2011, book by 5/31/11" so they do give you a decent amount of time to use the certificate. Or at least they gave me a decent amount of time to use the certificate; not sure if they update the book by/use by dates each month to reflect time going by and mine are just dated that way because my renewal is in October/November?
As for what happens if you don't send it in or if you book outside the marked travel period: I think this is going to be one of those YMMV areas. I've usually dropped mine off at the airport ticketing desk rather than mailing it in, and every year they look at me like I've got three heads. I'm pretty confident that at least some of those certs never found their way back to home base but I've never had a problem with the bookings. I'd think they wouldn't let you ticket if you didn't meet the rules of the promo but I think there are so many hands in the pot on this, that nobody "owns" the task of end-to-end validation. So basically if US is just going to send that cert back to Barclays in the end, nobody on US is telling Barclays to look for a cert from you, and nobody on Barclays is telling US to look for a cert from you. Same with the rules. Nobody at Barclays is telling US that your purchase has to be made within X date range and nobody at US is telling Barclays that you used the cert.
I honestly don't know what advice to give the OP as it seems like sending it in and having the date problem noticed is equally likely as not sending it in and having the absence of a cert noticed. I do think the chances of either situation causing the itin to be voided are pretty low but who knows....