It seems that lion-g beat me by a micro second, in Singapore we are early risers!
A few thoughts -
.1. V1 must not exceed Vr, but may be equal to it (the B727-200 is a good example),
.2. The maximum V1 used must (of course) allow for Accelerate-Stop within the available ASDA, and allow for continued acceleration to Vr and climb to 35' within the available TODA, both of these considerations being embodied within the 'total package' of Takeoff Performance requirements,
.3. The maximum V1 must consider Brake Energy Limits in the event of a rejected Takeoff (Vmbe),
.4. The Maximum V1 must consider tyre (tire for the Yanks) limit speed, although this will in the majority of cases be governed by Vr (Remembering that V1 may equal Vr).
There's plenty of other considerations for the LOWER limit of V1, not addressed here.
Regards,
Old Smokey