Later, always buy later. When leopard comes out you should wait for the next update because it will have Penryn - after that you should wait for the new MBP redesign - by then Intel will have a new mobile CPU out and it will be even faster. Wait, and eventually you will keep waiting forever.
The current MBP is pretty nice and October is still far away, updating to Leopard will cost you $129 at most. So if you find $129 more important than 4/5 months waiting, wait. If you find 4/5 months more important than $129 buy it now.