For new buyers, they are currently selling Feb (and now possibly March) use year. First allotment of points will come in 2009. Thus, if you buy a Feb or March use year, you will be able to do what you suggest, bank 09 points, borrow 2011, and take May 2010 trip using three years worth of points. However, understand that GVs anywhere go quickly during the 11 month window and that will likely hold true for BLT and thus to have chance of getting it you will need to call exactly 11 months out from date of intended arrival at 9 a.m. Eastern when Member Service's open. Morreover, because of the scarcity of GVs, no one should buy expecting to always be able to get one.
Note, if you are already an owner and buy BLT, you will get same use year you already have, and if that use year is not Feb, Mar, or April, you will not be able to do what you suggest because first allotment of points will be 2009 points in your existing use year and with June or after use year, you will still be in 2009 use year for a trip in May 2010, i.e., you will only be able to access 2009 points and ones borrowed from 2010. Nevertheless, that can be avoided if buying 260 points and you request a new buyer use year and thus go to having 2 use years; they allow that if buying the same or more than the minimum points required of a new buyer