Thanks for your notes c1i77, they were very helpful.. I have now migrated from Dallas to Atlanta without a hitch (thanks Tom!). Here are a few notes to supplement the ones above..

The new hosts seem to be about twice as fast as the old Dallas hosts, judging by the significant drop I've observed in the dashboard CPU graphs (my Linode is still running exactly the same services and under the same load).

It is a good idea to run a grep on your entire /etc directory for your old IPs before migrating. I had bound a number of services (apache, mysql) to specific IP addresses and had also configured manual networking - all these things needed changing.

The filtered ports at Atlanta are an issue. Be sure to check the list before migrating, and remember that they apply to both incoming and outgoing communications.

Last (and probably least), there don't seem to be any nameservers provided by Linode at Atlanta, the nearest being at Dallas (only 30ms away anyway). I guess we could use public ones instead but I have yet to look for any of these.