incr/decr can overflow the long return type

I'm currently using the Couchbase Server 2.0 Developer Preview, but I think what I am seeing is the same as Membase Server.
I'm using spymemcached in Java and I'd like to be able to increase a numeric key and return the new value. This works when the numbers are small as the result of the asyncIncr method fits into a Long, but when it doesn't Java overflows the Long and I end up with negative numbers. I can retrieve the correct (positive) value by doing a normal get as it returns the value as a String.
Is it possible to retrieve the result of asyncIncr as a String as to prevent this problem?
I'd also like to ask why negative numbers aren't supported and if there is a standard way to achieve this functionality by using an offset in my application code?

http://www.couchbase.com/forums/thread/large-and-negative-numbers