Length limit of 500 on primary keys?

[Shankar Unni]

Why is there a limit of 500 bytes on the primary key? (MySQL-Max-3.23.43 on 
WinNT).

Is there a way to raise this limit? We have some tables with columns that 
are VARCHAR(200), and need to make a primary key based on combinations of 
these columns (in some cases, upto 4 columns), and MySQL complains for all 
of these table definitions that "Specified key was too long. Max key length 
is 500".
--
Shankar Unni.