I won't be rooting my s2 until its at least a year old. Like to keep my warranty while the phone is still valuable!!
I just hope samsung doesn't ruin ics too much. Googles done a fantastic job with the ui.
Rooting only voids the warranty if you take it in rooted. Most people just flash the stock ROM/firmware back before returning it. In fact, most providers require you to at least try a "repair" or "restore" on the phone before sending it in. Which would remove any rooting or custom ROMs.