I have been bitten by this quite a few times before, so I started preparing in advance a .xsession file. Looking at this thread, I decided to check how things are now with -current and installed a fresh overnight build under VirtualBox with full configuration during sysinstall, enabling xdm and creating a new user. Everything worked as expected, the three default processes were started properly, I could checkpoint a session etc. So whatever the reason was with the earlier versions, with 8.99.2 it seems just fine.
The only thing which didn't work was first login with the created user - it didn't get the password - but I was able to login as root - again with xdm - and reset that password, after which the user login also worked as expected. That could have been a pebcak, though.
Chavdar