Yes, the money for the HBP (up to $25,000) needs to be in the RRSP for a minimum of 90 days before it can be withdrawn.
My suggestion is, if you have the cash and don't need it, dump it into the RRSP and put it in a money market or cashable GIC. That way you'll get the contribution receipt that you can use whenever you want.
That way if cash is a bit tight after you buy the house you still can get the refund in 2010 and beyond once your student tax credits run out!