I hear this a lot when I tell people I'm preparing to buy a new computer:
"Why buy a new computer when, as soon as you buy it, it's going to be obsolete? There's always something newer and better around the corner."
Have you heard the same thing? What is your response?
Most people who say this don't actually understand or fully comprehend the concept of the word "obsolete". In short, just because a newer or updated model of something comes out, does not automatically make the previous model(s) 'obsolete'. It simply makes the previous model... well... older, since it was the previous model. Older does not equate to obsolete.
In the case of Apple, and the majority of the updates they make... a mere 100-200 Mhz increase in CPU speed, a few GB more of HD space, increase to base model RAM, etc is hardly a reason to wait. The effects of such improvements are minimal and marginal at best. And if you waited, you only ended up wasting another six months with no new system. "Waiting for Leopard" is also a weak excuse, since one can always purchase the OS later and run it on the machine they just bought yesterday. Sure, you would save the cost of buying the OS separate, but when you factor in that some people have been waiting for over 6 months now, is it really worth 6 months or more of your time to save a mere $130? I don't think so.
Also, people who really live and die by that rationale would never actually buy anything, since their logic dictates that no matter when they buy their machine, a new version is coming which would render their purchase useless, thereby making the process of buying a computer an exercise in futility.
So, all one should do is simply assess his needs for a computer first, then buy the best model currently available to meet those needs. Being mindful of possible advancements is a good thing, just don't obsess over it and play the silly "waiting game" that so many do.