Sitepoint features a blog entry about PHP Predictions for 2005. Some are very doubtful (like Perl, Java and PHP developers uniting and XUL becoming mainstream), but others are very likely to occur.
I hope that the predictions about PHP5 adoption hold true. However it will be extremely hard, from an administrative point of view, to upgrade from PHP4 to PHP5, when you cannot be sure that you will not break thousands of customer-scripts.
While I believe some better known PHP applications will do a version bump and begin to require PHP5, it is likely to depend on the overall adoption of PHP5, rather than the need of the features it provides. A project is after all nothing without its users.
I hope to be able to upgrade the servers, which I manage, to PHP5 in 2005, maybe one at the time to see if it can be done without breaking too many scripts. However before applications like WordPress starts to fully support PHP5 (not require it, just support it), it will be tough.