The emulated standard input is blocked when I do the following:
1) Start DosBox
2) Select the main DosBox window, hit Alt+Break to break into the DosBox Debugger
3) Select the main DosBox window while the emulator is halted and type a key
4) Select the DosBox Debugger window and hit F5 to resume the execution
5) Select the main DosBox window, try to type anything. At least I can't, unless I break into the Debugger, type a key while the Debugger window is selected, then resume the execution.
I've used self-build 32 bit DosBox from 0.74 sources and from trunk 3789 sources. Build using VS2010.
As a side note, shouldn't the Debugger window drop all input entered while the emulation is running? Select the Debugger window while the emulation is running, type anything you want, select the main window and hit Alt+Break. The Debugger window registers the previously typed keys.