UPDATE 20th May 2011.
I'm very happy to announce that this problem seems to have been fixed in the 10.3 version of the Flash Debug Player in Windows. I'm also happy to say that Adobe did indeed contact my colleague Alejandro today to ask us to check again or provide more details. Nice.
Since Flex 4 and Flash Player 10 were published, we have been getting crashes when running the external debug Flash Player from FlashDevelop, somewhere after release 10.0.32. As workaround we just reverted back to the Flash Standalone Debug player that we knew worked, and carried on. Recently we wanted to move to Flex 4.5, and this became more of an issue as we need to now use the 10.2 player.
Originally we were pointing the finger at FlashDevelop (because of past posts etc. on this issue), but we have established that this is in fact a Flash Debugger Player bug and we know pretty much exactly how it happens. The fact that is has nothing to do with FlashDevelop.
If you are using resource bundles in Flex, and you have a file with keys and values that exceeds a point just over 12k, and you compile these files into the SWF using the locale=en_US (or = whatever) compiler option, this crash will start happening.
The workaround is to create a resource module swf of your resource files and load that at runtime, and then the crash stops (see this adobe article). Alternatively, you can break your resource bundle files up into less that 12k each. Please remember that this only seems to apply to the external debug player (projector).
We have registered this bug with Adobe - FP-6790 at https://bugs.adobe.com/flashplayer/, but because it relates to "a potential security issue", we couldn't see it or review it afterward!
Really hope this helps somebody with the same problem. This was true for Flex SDK 4.5 first release build and Flash Player Debugger (projector, sa_debug) 10.2 r159.