Blog Symbian Freek lists the following enhancements in the N93's firmware upgrade:

Out of Memory
Camera buffer changes for more free RAM
24.7MB free RAM after boot-up (18.8MB in PR1)
Out of Memory framework threshold changes

Camera
Change default setting of Optical zoom for video to off
Camera application freezes in low memory situation
Fixes for a few potential crash in camera application

Other SMS freeze
Uncompressed bitmap taken into use and WServ fix
SMS sending delay, hangs fixed
Call crashes fixed
Probability reduced by having uncompressed bitmap/wserv/RAM changes

Operator/Region relates changes.
Does not read voice mail number from Cingular USIM