I love this! Great use of the SGN function. The 10x10 grid of characters is clever - gives you a total of 93 characters. Thought I'd have a go at porting this to the ZX Spectrum:
1 INPUT m: LET n=1+INT (RND*m): FOR t=1 TO 99: INPUT (t);"?";g: LET d=SGN (n-g): PRINT CHR$ (61+d): LET t=t+99*NOT d: NEXT t
Managed to get it into 1 line of BASIC using just 68 characters (gotta love the Spectrum tokenisation storing keywords in 1 character!). Not as pretty as the 10x10 grid, but kept the same gameplay as the original (unless it takes the player more than 99 guesses!)
Anyone fancy trying to make it even smaller on another platform? What about you Commodore lot?