With gigbit connectivity - you should be fine...
I would want to keep the I/O down to a minimum during a DMotion. DMotion gets it unoffical name from the "delta" files created on the ESX 3.x.x hosts VMFS 3 volume (where you moving the VMDK files too). Plenty of people here have reported problems if the deltas in snapshots grow beyond 2GB. So you want to do it as quick as possible - to keep the delta as slim as possible...
Regards
Mike
ps there's nothing more reliable - than a power off and cold clone to the VMFS3 volume. Make sure you have an effective VM backup in place before trying any cold-migrate or DMotion...
Regards
Michelle Laverick
@m_laverick
http://www.michellelaverick.com