While Lenovo does a very good job of using current components there's no way to predict just when certain changes will be made. There are simply too many variables involved in making a change to an existing ThinkPad model. Thus the question becomes very similar to the age old question "when is the right time to buy a new computer?"
What I might suggest, is to focus on when you "need" the computer and make your purchase then. I would suggest, that if you absolutely must have the most powerful graphics solution, you take a look at the W500. I use one for my 3D design and other graphics intensive applications and it's a great machine.
Cheers...
Workstations... T40p ~ T41p ~ T42p ~ T43p ~ T60p ~ T61p ~ W500
T Series........... T20 ~ 21 ~ 22 ~ 30 ~ 40 ~ 41 ~ 42 ~ 43 ~ 60
X Series........... X20 ~ 30 ~ 40 ~ 60 ~ 60s ~ 200 ~ 200s
