I used pg_migrator to migrate my rather large production databases
(couple hundreds GBs). Schemas are easy, as I keep datetimes as
bigints myself (for various reasons) I won't get the float/int
problem.
But I do understand, that keeping dates as floats might cause grief
(unless you create operator that compares timestamps with cast to (0),
etc). But that's another storry.
I wouldn't be afraid to use 8.4. I am using cvs head of postgresql in
testing al the times since 8.1beta , and never had an issue (that
wasn't my own fault).
But goes without saying, every release has bugs, so sooner you test
it, sooner hackers can fix them, if you find one. So my suggestion,
migrate data, do preproduction tests, and stuff. Once you're happy, go
for 8.4