As far as I understand they intend to convert all the internal bug tracking system, etc to FB/TB as soon as 1.4 "final" is out. However, it does not look very promising at this point that 1.5 schedule will be met. 1.4 is nowhere near to be released.
<a href="http://bugzilla.mozilla.org/buglist.cgi?changedin=10&resolution=---&field0-0-0=flagtypes.name&type0-0-0=equals&value0-0-0=blocking1.4%2B">Bugs</a>.
I guess it will take a month to move fully to FB/TB and only after that "real" development start happening, also considering that we have summer now, most probably a lot of developers will have some sort of holidays. I guess the best would be to postpone 1.5 "until it is ready". Thunderbird needs real work to be done with it to make it a good replacement of Outlook Express for many users. Also none of the optimization was done so far, afaik - since TB takes full 50Mb of ram (physical + virtual). And both projects still use their own GRE, even thought as I understand they should have one in common. Well, I'm not really expecting to see anything "working" until later this fall, but may thats a good approach, since both TB&FB lack some needed functionality.
My only wish is for them to make MIDAS work in FB as soon as humanly possible, also most of the recent nightly builds of FB are broken completely, not being able to display even simpliest of the websites. :( I had to go back to Gecko 1.4b releases.