Actually, the problem all of us are having doesn't have anything to do with Entourage. Its not an Office issue at all. It appears to have something to do with Snow Leopard and Exchange.
I have two clients who are also experiencing this issue. Both are using Exchange, both are using Snow Leopard. But only one is using Entourage (2008, fully updated of course). The other is using Apple Mail, and actually doesn't have Office at all...she uses iWork '09. The client not using Entourage has this issue chronically, and yet, when she attaches files to emails from her personal IMAP acct, no beachballs at all.
I'm experimenting now with my Entourage using customer, because I think the problem is Spotlight related somehow. In the privacy pane of Spotlight's preferences, I've added her entire hard drive to the exclude list. Of course, this means that she can't search her HD anymore. But she's OK with that for a few days if it means getting to the bottom of the problem. So I'll know in the next 24-48 hours of the problem is actually Spotlight related or not.
Of course, if it is Spotlight related, that's nice to know, but it creates a different problem because neither client is willing to do without Spotlight endlessly. So I don't know what will happen there. But for everyone here looking at Office as being the issue, it isn't. Its more global than that.
-oyster