I still have a few sites running on Drupal 5, and I'd rather upgrade directly to 7 when it comes out. I figure it's probably not going to be a supported upgrade path, but still, has anyone tried this (with the betas or RCs), and what should I look out for?
AFAIK, you cannot upgrade from 5 to 7 in one step. You will have to upgrade to the latest version of D6 first and then upgrade to D7.
With every major version of Drupal, the API changes and contributed modules come and go. That makes upgrading a more difficult task than simply replacing files and running update.php. You will sometimes have to choose which contributed modules you're going to upgrade to, and if you have custom code (modules and/or themes) you will have to adapt it. If you're upgrading from 5 to 7, you need go through that process twice.
Depending on your site and the amount of content and settings you have stored in your database, it may be easier for you to build your D7 site from scratch and then import your existing content using Migrate module.
Of course, regardless of the method you choose: backup, backup, backup before you do anything.
The man who says "It can't be done" is often interrupted by the man who says, "I JUST DID IT" :) To wit:
Nathan Haug: "Direct Drupal 5 to Drupal 7 Migration in 24hrs"