The biggest lock-ins are from Office formats, and Active Directory integration with network and communications services. Crack either of those and the bulwark starts to crumble rapidly.
I will agree with Office but I've a hard time seeing AD disrupted by anything else. The alternatives aren't better and/or are a support nightmare.
And with Office, it seems the Office 365 offering with the web clients has a very good shot at technically beating Google Apps... if they can sell it efficiently is another matter.
I'm not saying that AD is easy to disrupt (the primary challenge is that there's no one clearly superior replacement), but that if you crack it, a massive benefit / bonus of Microsoft's product line is voided.
And with Office, it seems the Office 365 offering with the web clients has a very good shot at technically beating Google Apps... if they can sell it efficiently is another matter.