From where, exactly, did you download the 6.57 driver? I just checked on the Epson USA support site and it's still at 6.11.
Regarding the original question of the thread: for printing RGB color images, I have not observed any compatibility issues between CS4, Leopard, and the 3800, when printing via driver 6.11 from the Epson USA site. I also print via Lightroom 2 to my 3800 without issues.
There are some known issues regarding printing in the Advanced B&W (ABW) mode, hopefully to be resolved once and for all, but these can be worked around.
If you look at Jim-St post above you will see the link to the new driver on the Epson UK website.
Below is the Epron's description of the driver:
Download Driver {v.6.57:} 28-May-2009, 44.03 MBs
Operating Systems:
Mac OS X - PowerPC, Mac OS X - Intel
Languages:
English, Français, Nederlands, Deutsch, Español, Italiano, Portugues
Products:
Epson Stylus Pro 3800
WHQL Certified:
No
File Format:
DMG
Additional Information:
The Advanced B&W mode (if available as driver option) and Color mode is improved with the driver option "No Color Management" and Adobe Photoshop CS4