Checking the options for dev_appserver.py, I couldn't find a setting to tell it to stop checking for updates to the SDK.
Since then, I've re-evaluated my decision and, a few weeks ago, I ported the Singularity web app to Django from webapp.
Note that your document class in Flash projects should extend org.gaeswf.flash.BaseApplication and your Application MXML in Flex should be of type org.gaeswf.flex.BaseApplication (if none of this makes any sense to you, see my Flex 3 Code Behind Quick Start.)
The updates in a nutshell:
How cool, I just heard from Brajeshwar on Twitter that The GAE SWF Project is currently the featured application on the Google App Engine Application Gallery.