This problem only affects those who have old hardware as far as I can tell. At least till Sun fixes Java's transparency rendering on linux.
The client looks the same on linux as it does on WinXP using Java JRE 1.5.0_06 and relatively new graphics cards on both. (Will test other versions of java later.)
However on an older computer with a ~4 year old graphics card it looks horrible, transparencies are white, with JRE 1.5.0_06, JRE 1.5.0_10 and JRE 1.6.0.