If you don't need them until August, wait until April or May to order. The vouchers that are being mailed out now expire in May 2003. Keep an eye on these boards, when the expiration date changes to 2004, I'm someone will post it. If you happen to get a voucher with the 2003 date, they will exchange if for you.
Check out www.mousesavers.com. Mary has an area on her site that gives all the details about the vouchers. Do a search here and on the budget board to get feedback on how the vouchers worked for other DISers.