Virtual screen with Intel GMA3100 ?

Last week I upgraded my development machine, from a AMD Athlon XP 2000+ to an Intel based one, featuring GMA3100-based onboard graphics. Everything's working smoothly that far, I have only one issue: "Virtual" keyword in the "Screen" section seems to be ignored. This was working since my very first Linux installation in 1996, so I'm really used to having a big virtual screen. This is on Slackware 12.1, using xf86-video-intel 2.2.1. Any ideas ?

Alex