We here in Exchange Hosted Services get a lot of traffic over our networks, but it's nothing compared to what they see over in Hotmail. I was talking to some of the guys who do some Hotmail spam filtering and the problems they have are things that we simply have not yet had to deal with.
Hotmail spammers are clever, and the clever ones specifically target Hotmail. What they will do is get into a range of IPs and start spamming Hotmail clients in huge bunches. Then, just when the Hotmail filters have caught on they will switch IP ranges and continue spamming, so adding a blacklist against that range is ineffective. They keep doing this IP-hopping trying to stay one step ahead of the Hotmail anti-spam team. Oh, and they spam from China...
Not only that, but spammers will register accounts on Hotmail and will send themselves test spam to see if it gets through. Once it does, wham! They start their spam-o-rama, hopping between IP ranges. It's very difficult to keep up with this type of spam because the spammers are intentionally trying to stay one step ahead of the anti-spam crusaders.
I haven't really seen that on Exchange Hosted Services (that I am aware of). I would guess that this is because we aren't big enough to start deliberately targetting. The volume of Hotmail traffic greatly dwarfs us. I guess once we start taking over the anti-spam world the spammers will focus their guns on us deliberately. And when they do, we'll be ready.