IBM has announced that they will stop supporting WAS 5.1 in September 2008.
The announcement is "Software support discontinuance: IBM WebSphere Application Server V5.1 products": Support ends for all editions of WAS 5.1 on September 26, 2008 (18 months from now). You can see confirmation on the support lifecycle page for all versions of WAS as well as for all IBM software products. WAS 5.1 has been available since January 16, 2004, so it will have been supported for 4.75 years.
Support for WAS 5.1 on z/OS had previously been announced to end on April 30, 2008; that has now been revised to September 30, 2008. See "Software service discontinuance: Selected System z products — Some replacements available." The support lifecycle pages don't seem to have been updated for z/OS yet. I don't know why the official date is Sept. 26 for distributed and Sept. 30 for z/OS; it's usually the last day of the month.
You can also check out the IBM Software Support Lifecycle Policy summary and details.
So what's the point of these announcements? If you're using WAS 5.1, you need to start planning to migrate to WAS 6.0 or 6.1 (and be finished by September 2008).
WAS 5.1 Support Ends September 2008