As a host moves around, it may be assigned different IP addresses based on its location. When this happens, the host acquires a new IP address and continues to operate, but the previous TCP connections are invalid and torn down.
Normally this process takes just a second or two, and presents no problem to most applications (watch Outlook as you roam and you will see what I mean; the connection will drop but then reconnect on its own)
The problem is applications that cannot handle a 1-2 second drop. What applications can you think of that fall into this category? I can think of VoIP and streaming media as about the only two right now.
Mobile IPv6 was designed to eliminate this 1-2 second drop by creating a "Home Address" that doesn't change, regardless of location. The changes necessary to implement Mobile IPv6 are not insignificant, and for the limited number of applications that truly need the fast failover time, we just weren't able to get MIPv6 included into Windows Vista.
Do you have a killer scenario where you think only MIPv6 will work? Leave a comment and let me know!
By the way, check out http://www.microsoft.com/technet/community/columns/cableguy/cg0904.mspx for a terrific summary of Mobile IPv6.