I have had Slimserver 6.2.1 (2005-10-31, 2005-11-01, 2005-11-02 releases) 
running on a machine that uses Vector Linux (Slackware based).  Tag fields were 
created in Windows using Foobar.  Tag fields are of this form:
ARTIST= Brad Shepik; Scott Colley; Ton Rainey
ARTISTSORT= ShepikBrad; ColleyScott; RaineyTom

If I set the one album with these fields as my music library in Slimserver 
6.2.1, with ';' defined as the value separator, then in Browse Artists the 
artists are displayed in this order:
Ton Rainey
Scott Colley
Brad Shepik.

Order is always the same.  When I used my full music library, I got a similar 
apparently random effect, though again the order in which artists appeared was 
always the same.

If I delete the ARTISTSORT field, the Browse Artists display is
Brad Shepik
Scott Colley
Ton Rainey.
So they are correctly sorted alphabetically according to the beginning of their 
name.

In 6.2.0, with the ARTISTSORT field in place, I get the order I want:
Scott Colley
Ton Rainey
Brad Shepik

In 6.2.1 when I search by artist, each artist can be found using either his 
first or his last name.  In 6.2.0, only the last name shows up on a search.

I haven't tested for the bug on other OS's or platforms.  I also haven't tested 
whether the bug appears only when there are multiple values in the artist and 
artistsort fields.