For the next upcoming Mac OS X release, Mountain Lion, the oldest models supported (depending on type) will be those released around 2007-2008.
I have an Early 2008 MacBook Pro which I use as my daily machine, and it’s still going strong. I’ve maxed out the memory and replaced the HDD but other than that I don’t have any immediate plans to replace it. Looks like I just squeezed in under the wire though to get Mountain Lion support, so an upgrade maybe on the cards sometime in the next couple of years.