I have a 1st gen MBP with no plans to upgrade soon. It is still half decent, performance-wise and nothing's broken. I did upgrade the wireless to 802.11n and the hard drive to 500GB.
I have been trying to buy an iMac for when I need more performance and screen space, but I can't get one with a non-defective screen. If I have to give up on the iMac, I guess I'd think abouit a new laptep. But I'd really be looking for a significantly bigger jump in capabilities than is currently available and I don't know when that will be practical in a laptop. E.g., I'd want a 300+ GB SSD + quad core + a really fast external interface (USB3, lightpeak, whatever) and not go broke. So 2011 is probably the earliest I'd replace my MBP.