Hunting for buffer overflows

Is there a way to use a debugger (immunity preferably but I'll take gdb or any other debugger you know how to do this with) so that any time a piece of memory holds a particular value (say AAAAAAAA) the debugger will make a note of what line it happened and then continue on until it fully crashes.

Not just a simple watch but anytime any register or memory in use by the program contains an A to make a notation of this.  It seems like a really good idea that would take some of the hard work out of exploit development but I have yet to see any straightforward talk of this.

You'll have to modify something like Bochs or other CPU emulator to catch all memory and register writes. It will be *very* slow.