According to this post on Mike's blog today, it looks like "PHP 5.1 is around the corner" (at least according to the latest copy of the news file).

Here's a tidied up excerpt of the current NEWS file with all relevant changes since PHP 5.0, which may help on the decision to upgrade (even from PHP 4).

Some of the things mentioned include:

• More than 350 fixes
• Some extensions moved out to PECL (11)
• PEAR upgrades
• Upgrade of the bundled Windows libraries
• Loads of Improvements and Addons

You can check out his post for the complete list...