Buy what offers the best price/perfromance ratio for you now. When you'll see something better after one year or so, then go for that, if you can afford it. Don't play the waiting game, since is not going to take you anywhere. You'll eventually find yourself with an old platform and poor performing cpu and when you do decide to change it, something better/faster will be right around the corner. This will always be the case.
The backwards compatibility, that you are speaking of, is crap. Look at how many problems Phenom cpus have, when installed into older AM2 mobos, like limited or no overclockability, lower HT speeds and many more. The same thing on the LGA 775 side. How many of the "old" P965 mobos ( the overclocking stars of the core 2 duo release) are doing great with newer Penryn chips? Few, I guess. You still have to buy at least a P35 to get everything out of a Penryn, like you still have to get one of those 790GX chipsets, with that sexy new south bridge, that can take the Phenom at more then 3 ghz and squeeze everything out of it.
AM3 cpus will require AM3 mobos to work great, like myocardia said. This is the awful truth....