Some people, myself included, still hope that shortly after the introduction of Snow Leopard the MacBook Pros get a slight spec bump.
For the 15" one would hope a slightly higher screen resolution, something between what it is currently and the 17" resolution.
I see this happen especially if Snow Leopard finally implements Resolution Independence.
There's also hope that the 15" might inherit the 17"s 8GB max RAM limit as the nVidia chipset supports it technically.
And last but not least the 17" might get a Blu-Ray drive as built-to-order option.
Again, only if Snow Leopard supports Blu-Ray, which I kind of expect it does.
When will this happen?
My guess: as early as June, as late as November 2009.
I think it will also depend on when Intel will deliver Nehalem notebook CPUs. If those are ready by January 2010 Apple might push the next small update forward to have a wider time gap to the Nehalem version.