Normally my rule is to never wait cause they're will always be something newer, better and cheaper coming out right around the corner. But in this instance, I say wait.
Haswell is supposed to start production late on Q4 of 2012 which is right about now and be out for retail around Q2 of 2013. That's not that long a wait.
Now the main reason I say wait is the rumor that just sprung up that the next generation, Broadwell, wont be available to buy separately and will only be available soldered to the motherboard. Desktop CPU's for that generation will be stepped up Haswell procs which likely means they'll still work on Haswell socket motherboards and that means you can keep your Haswell rig a generation longer. Drop one of the refreshed Haswell chips in your rig in 2014 and you've got a lot more life without have to swap in a whole new mobo and OS and all that.
Now all that said, it ultimately comes down to what you're running now. If you're on a AM2 dual core then you might as well jump now but if you've got something solid like a i7-920 or even Core 2 Quad, then it might be worth it to stick it out a little longer at least until we get a little more info on those Boradwell rumors.