No it's not. I've supported well over that number. The key is you have to buy the right hardware up front. Then Exchange runs like a champ. But it also requires a sound AD implementation.
Remember, I'm comparing to UNIX solutions. I've always had a few questions, like why can't I have all three nodes in a three-node cluster active? I'm forced to have one idle. What a waste. And do you realize it uses the JET Blue (MS Access) database engine? Ick!
If Microsoft wants to impress me, allow a full Exchange cluster, and have it use an SQL Server cluster as the back-end. Then both the data and the application will be in full failover.