We have an application to move to google app engine, which the owners require to continue to use incrementing numbers, or an approximation thereof, ie it would be ok if each server has a block of 100 or so numbers to dish (sharding).
Is there a library for this or is it still roll your own?
Roll your own. But that's a highly unscalable requirement; you won't be able to get more than about one number per second in the simple case (a singleton counter datastore entity).
There are solutions if you have wiggle room for 100, such as a sharded counter allocator.