The message may also say something line:

Unable to update app: Error posting to URL:
409 Conflict

Another transaction by user Your.Name is already in progress for app: your-appName, version: x. That user can undo the transaction with "appcfg rollback".

Basically this means that in the midst of your previous deployment of the application, the process was interrupted. The Google SDK interprets this as a deployment that must be rolled back. There is no resolution to this without locating where your \bin directory containing the or appcfg.cmd file exists in order to rollback the corrupt deployment.

Since I was working on a windows box when this happened to me, here is where I found the appcfg.cmd file:

C:\Program Files\Eclipse\eclipse\plugins\\appengine-java-sdk-1.3.8\bin\

Look for your application--more specfically, look for a directory called WEB-INF. In my app, WEB-INF resides under the \war directory. Here it is as an example:

C:\Documents and Settings\Administrator\workspace\StockWatcher\war

Open up a windows command window or execute cmd from Run (Start>All Programs; Run; type: cmd).  Execute the command:  appcfg rollback your-appName by wrapping the location of where the appcfg.cmd file exists, and wrapping the location of root of where you found the WEB-INF directory. This should correct your  previously corrupt deployment. See the screenshot below as an example of the command line and of the expected message displays. BTW, if you don't work with dos windows much, remember to include the quotation marks "" as you see them. Windows needs the quote to deal with "Documents and Settings" and "Program Files". If you forget the quotation marks, you may see an error message that indicates a file or directory is not found.

 Eclipse Google App SDK deployment