Nehalem CPUs will introduce two new sockets; socket B for the desktop and socket H for servers.
If Intel can manage to release a new CPU architecture every two years (as they stated they would when Conroe was first released), then you can expect socket changes every two years. It's not definitive, but it is very possible.
Therefore, don't depend on "future proofing" 'cause within about two years whatever PC you build or buy will become "obsolete". That's 'cause your motherboard will not be compatible with the next gen CPU.
There's a flip side, perhaps the successor to Nehalem will still be socket B and H, but it will require a new chipset.
If you can wait until Nehalem is released in the second half of 2008 (my guess is Q4 2008), then wait.
If Q4 2008 is too long, then consider Q1 2008 for the Penryn CPUs.
If waiting 5 months is too long, then build your PC around Conroe now.