I have a workflow rule that fires a time-based action (an email alert and field update) and this rule has been working fine, firing as expected thousands of times over the past couple of months since I created it.  This morning however, the workflow rule was due to fire on 4 records and bombed out (did not fire) on each of them.  I received four similarly worded email messages:

Subject: Salesforce workflow could not perform a pending action

Details:
Object Type: Drip Event

Record: DripEvent-014794
https://na2.salesforce.com/a0E40000001KG3S

Workflow Rules attempted: Healthy Kids Campaign Email Series 2
https://na2.salesforce.com/01Q40000000FMKj

Workflow Actions attempted: Healthy Kids Campaign Email Series 2
https://na2.salesforce.com/01W40000000D09T
Mark Send Completed
https://na2.salesforce.com/04Y400000000CgI

I experienced issues with time-based workflow when daylight savings time hit this past fall and the guidance from Salesforce  was to completely rebuild all of my workflow rules (which didn't correct the problem by the way, my workflow rules were still trying to send at 11:00pm the night before they were supposed to, and failing to fire).  My question -- is there a known issue with time-based workflow rules following the Spring '09 release?  I've looked over my workflow rule a dozen times today and nothing at all has changed in our Salesforce instance relating to this rule.

Thanks much!

Dave