Note that falling back to DNS updates means minutes of delay.
Yes, if they move simultaneously, they will get minutes of delay (due
to DynDNS update), and the TCP session will break.
That's not a problem. User's experience of the delay is, which is
why Mobile IP has servers of home agents as a stable point to
receive packets.
Note that, as you assume NAT (not very elegantly at the application
layer), mobility must be port wise (which requires NAT transparency
at the transport layer), because access points may be behind NAT.
MOPS (Mobile TCP) - as defined in my spec, will work fine through
source NATs (Home routers), assuming the Mobile Node is well...
mobile.
But it doesn't cover load-balancers (a.k.a. destinations NATs), and
doesn't cover if mobile node client is behind NAT, but the server has
moved (Home Router becomes a destination NAT, on traffic from
Internet).
--
-Alexey Eromenko "Technologov"