Normally, rooting guides for the S2 involve flashing a kernel to the device to install busybox and su. This xda post suggests a method for doing so via adb (found approximately in the middle of the first post):
adb shell mount -o rw,remount /dev/block/mmcblk0p9 /system adb push busybox /system/xbin/ adb push su /system/xbin/ adb shell chown root.shell /system/xbin/busybox adb shell chown root.shell /system/xbin/su adb shell chmod 755 /system/xbin/busybox adb shell chmod 4755 /system/xbin/su adb shell /system/xbin/busybox --install -s /system/xbin adb shell mount -o ro,remount /dev/block/mmcblk0p9 /system
This also seems to be the method tools like SuperOneClick use. Now I recently found this blog post indicating the above method does not increment the hardware counter in the S2 that counts flashes of custom firmware. More than that, it can also be used to install ROMs via Mobile Odin, also without increasing the counter, because they come form the device itself and not from a computer via the USB cable.
I fear to try out myself and lose my precious counter value of still 0. Has anyone tried out a method like this before and can confirm that it really does not affect the counter?