As someone else said, the 15' upgrade will be a relatively small upgrade so there's little reason that we know right now for you to wait. To boot, I don't really see a release happening much before the end of May and it could well be mid-June at the very earliest if Apple decided to wait until the dual core ivy bridge processors are also release so that the entry level 13' can be updated at the same time (very likely unless they decide to go quad across the range). So if you need the machine before mid-June, I would buy it from apple.com, then at 14 days and no less ask for the machine to be returned and make arrangements for the courier to pick up a week or two later (say you are on vacation or something). I don't think Apple really cares when the machine lands with them. It's when you initiate the return that has to be within 14 days. That may buy you an extra few days and get you closer to the release date of the new range.
That been said, if the 15' machine changes form factor, you will be kicking yourself for not waiting. In general, pay little attention to those who simply condone and good luck with whatever you decide to do. Remember these same people people are on this board because they also actually fret about release cycles (why else would they be on mac rumors website).