There have been reports of Screebl not starting at bootup on some devices.  It turns out that in most cases this is related to Screebl having been moved to the SD card.  On some custom ROMs (e.g., Cyangogen) apps can be moved to SD regardless of whether the application developer has indicated that the app may be moved to SD.

In Screebl's case, a background service is required to run.  This is not possible to do when the app is being auto-started at phone boot time, as the SD-card is not mounted.

The bottom line is that if you move Screebl back to phone storage, things should start working for you.

Thanks to "trevor" on the Screebl Google Group for identifying the solution to this problem!