Ummm...first why on earth would you switch from Postgres to MySQL - it's a big step down?
However, if you are forced to do this because of a change of ISP or something, then I think depending on your coding, it will be very difficult.
MySQL doesn't support subselects, custom types or operators, stored procedures, foreign keys, sequences and only vaguely supports transactions. It also has highly non-standard date and time manipulation.
So, if you've used any of the above you're in trouble. However, if you've stuck to the basic INSERT, UDPATE and DELETE syntax you won't have too much work to do.
Chris
