The bset way round this problem is to keep your current (pre W7) OS also up to date -- apply updates, install the same software on the old OS as W7 etc etc. - This is for VISTA installations. For XP the only way to upgrade is first Via Vista and then to W7.
Now if you don't want to keep a separate machine for the old OS -- either "Image" it or "Dual Boot".
My preference is for "Imaging" - if you Dual Boot --especially with a Beta test OS and something goes wrong then you are Royally hosed up as it's 2 OS'es that'll need fixing.
It only takes around 30 Mins to restore a full (with loads of Large apps such as Office 2007, Photoshop etc etc) VISTA ultimate X-64 35Gb installation so smaller installations will take a lot less time.
So when W7 comes out for real you can just "upgrade" your previous OS.
In some cases Upgrade is a viable option -- however for others it could be better to just bite the bullet and re-install.