Jhbuild seems to have a general problem with D-Bus. This has become obvious now that GConf is using D-Bus instead of ORBit (see these gconf-list messages: 1, 2).
To make GConf work in jhbuild, you need to do this in each jhbuild shell:
dbus-launch --exit-with-session bash
Then there’s the problem of Avahi. To make Avahi work in jhbuild, you need to do this in each jhbuild shell:
I wish that jhbuild has some general solution for these issues by default because I suspect that these are just hacks.
However, I imagine that it’s a difficult problem. For instance, you probably don’t want to kill the main (non-jhbuild) D-Bus (and D-Bus services) because that would break your desktop. But you probably want a different D-Bus (and D-Bus services) for jhbuild. The two prefixes don’t seem to work well together in D-Bus at the moment.