Lots of folks argue that you should spend lots of time researching, decide on focal lengths, then go buy the best lens on the market. For people that have been avid photogs for a long time, I guess I'd agree with that. If you're going to end up spending the money anyhow, save up and spend it once and you'll be ahead in the long run.
Let's face it - most people, even those who own dSLRs, don't spend a bunch of time on the internet talking about their hobby. They're out using their cameras instead.
In my case, I did some research, but due to budgetary constraints I went ahead and bought the Sigma 70-300 APO shortly after I bought the 300D. It was less than $200 shipped, and it gave me the flexibility to shoot a LOT more photos than I would have if I'd saved up for the 100-400 L IS. Now that my budget is much better, I'll sell the Sigma when i buy the L glass, and probably get 1/2 my money back -- not bad, being able to "rent" a lens for 9 months for $100. Along the way, of course, I've taken well over 3000 shots, the majority of which used the 70-300.
Using a variety of cheap gear also taught me a lot about what I do as a photographer, and therefore how I should invest my resources in the future. For example, I've found that the 18-55 kit lens is plenty wide and sharp enough for everything I ever shoot, so I don't need to waste time with a 17-40 L lens. Also, I do a fair amount of lower-light or flash restricted shooting, so a 70-200 f/2.8 is going to be a must for me down the road. I learned that years of abusing caffeine, combined with a genetic disposition, leaves my hands a bit shaky so IS technology is a must for longer focal lengths.
Finally, I'll point out that once you get closer to the $1,000 price level, it's very easy to get 80% or more back out of your lens purchase price when it comes time to pull the trigger on the upgrade. If you ever need to, that is.