I have two identical systems (Intel S975XBX2 workstation board w/E6750 core-duo processor, 2G DDR2 and ATI Radeon HD 2400 video, except one has an EIDE boot drive and 4 SATA dirves attached to system board controller with software RAID & LVM (this system works well with 8.04) and another with Adaptec 5405 hardware RAID controller with 4 SATA drives configured as 5 RAID volumes. This system works fine with 7.10 but hangs on a blank screen after login with 8.04, both after upgrade from 7.10 and with a clean install, and both Ubuntu 2.6.24-16.30-generic and Ubuntu 2.6.22-14.52-generic have the same problem. I just reinstalled 7.10 with SSH-server and SAMBA, then upgraded to 8.04, and while the system still hangs at a blank screen after login SAMBA and SSH-server are working, and I have acquired log files. dmesg.log ends with the same error message in both Ubuntu 2.6.24-16.30-generic and Ubuntu 2.6.22-14.52-generic, I have attached dmesg.log from Ubuntu 2.6.24-16.30-generic.
The computer with the problem runs 8,04 just fine with the Adaptec 5404 controller removed and an EIDE drive installed temporarily as a test.
I have a machine with hardware raid which appears to be showing the same behaviour. I have managed to create a partial workaround by killing a dbus-daemon process, so I have logged a bug against dbus in case that is where the real problem lies. It's bug #229637.