I asssume that teh S80 is the same as other Volvo's, and if so, the pin has two functions: it secures the brake rotor, and it acts as a a positioning pin to be sure that the wheel is always re-mounted in the same position. Lots of european cars have this positioning pin, and a corresponding small hole in the wheels between two of the stud holes. Volvo steel wheels have a hole, while many of their alloy wheels have a hidden recess that you can only see from the back side. Check your stock wheels and you should find it. Sorry, don't know if you can just grind down the pin, probably depends on what clearance the new wheels provide ( you can't grind it flush since then you couldn't remove rotor) . Check that the new wheels don't already have a recess and you hjust have torotate them to the right spot where they will drop in.
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Mike Sullivan ('91 745 (171k), '93 965 (147k), '95 855T (60k), '98 V70XC (78k). Past Volvo's: '85 744 (256k), '86 245 (165k), '86 245 (195k), '88 745 (208k).