I feel it's just as likely that the MBP will be updated in October/November as in Spring. The MB is rumored to be released/updated soon. All of the technology expected in the next MBP is now or will be shortly ready.
Arrandale CPU - ready soon
Nvidia 200m or other graphics - now ready
USB 3 - now ready
BluRay - been ready and Apple seems like it has accepted people want with rumored support in iTunes and QT
In addition, Apple seems to be updating more frequently given it needs to so buyers justify updating now instead of later in bad economy. For Apple to keep MBP sales up during holidays it will need to update. I actually think we will get more frequent spec bump updates for the next several years as Apple realizes sales soar with each bump. A BluRay capable Mac for the holidays would justify purchases for many.
If you need it now, buy it, but if you can wait I believe you will be rewarded.