Just as a side note, every airport express that is part of a "roaming" or "wds" network needs to have access to DNS server in order for Back to My Mac to function.

This is in relation to Back to My Mac and "roaming" or even "wds" airport base stations.

If you follows articles such as http://support.apple.com/kb/HT4260 or http://support.apple.com/kb/HT4262 to setup a roaming network, what they leave out is that inorder for Back to My Mac to work you need to have the "DNS Servers:" field filled in with valid DNS servers. This is more of a problem for "Extended Wi-Fi base stations" because "Primary Wi-Fi base stations" can have their DNS server automatically assigned by your ISP.