Yes we are aware of this bug. I wrote the bootloader that is in the mega. I have fixed the bug and the fix has been submitted. If anyone wants the lastest bootloader source and cant find it, send me a private message. I can also send you a hex file ready to program into the chip
The bug is ONLY in the 2560 bootloader (stk500v2) not in the optiboot which is the new uno.
The 2560 requires st500v2 protocol to be able to handle above 128K.
Lefty is correct, spaces will fix the problem. The bootloader watches for "!!!" (no spaces) to switch to monitor mode
For more info about the bootloader and what the "!!!" does, refer to
http://www.avr-developers.com/bootloaderdocs/index.html
Mark