Dreamhost does not provide guaranteed static IPs. It may not change for a while, but it can and will change without notice at some point. It may have a private IP, which is reserved for your use only. However, even private DH IPs are not static. This makes it easier for them if they need to do some network restructuring.
There shouldn't be, so long as you set up the new MX records on dreamhost to match the same as your old host BEFORE you change the name servers. Do that here:
Add the domain with full hosting. Opt-out of Google mail. Go back to manage domains and change the MX records.
That way, as the nameserver change propagates, the MX records are the same regardless of the progression of the propagation.
For more info, see: http://wiki.dreamhost.com/Configure_hosting_on_a_non-DreamHost_domain