Although BAM is up and running, my hosts have not contacted BAM since. I have manually sync'd two of them, and they now talk regularly.

Heres my output from http://www.boincstats.com/bam/host_list.php as at 06-03-2007 08:05. The last three have not said hello for ages, although they are still happily processing tasks.

29907 5.8.11 06-03-2007 / 4:00:39
30162 5.8.11 06-03-2007 / 4:39:09
30234 5.8.11 03-03-2007 / 14:17:01
30302 5.8.11 04-03-2007 / 5:18:21
30339 5.8.11 04-03-2007 / 3:00:06

It would appear BAM needs kick starting after extended downtime?

I think that by default (this is implemented so in BOINC CoreClient) if a computer fails a contact with the AMS, it will retry its communications with the AMS after 24 hours, unless a user manually synchronizes its computers.
And I see that none of your computers has reached this threshold, so I guess this is the case with your computers. If you don't want to wait for 24 hours you can manually sync them and then the communications with BAM! will continue as usual.

Son is correct about the 24 hours.