Thank you for reporting the issue.
Could you have please have it in our database http://bugbase.adobe.com with all the required files to reproduce the issue.
If possible, please attach the sources as well, it'll help us reach the root cause quickly.
I tried a brand new, no content, mobile project in Flash Builder and got the same result when it tried to package.
I did solve it though. I had Java 6 release 30 installed on my machine, when I installed Java 7 release 3 I was able to package properly without the error.
I couldn't find anything about a proper Java version needing to be installed, but it should get added to any existing documentation.
Glad to hear that David, It would be really helpful if you could get us some information on following.
What type of project it was (Flex Mobile, ActionScript Mobile etc?) on Flash Builder 4.6?
Is it a 64 bit machine? It's Mac or Win, which version of OS.
What version of AIR SDK version you have in your Flash Builder. (Goto Flash Builder directory\Adobe Flash Builder 4.5\sdks\4.5.1\bin\ and type adt -version on command line)
We assume that you're using Slow packaging.
Thanks a lot.