I have the same problem, and I've had it since I bought my 3870. My old X600 worked perfectly with the same hardware.
It only happens with the fglrx driver though.
See this bug at launchpad for more frustrated users: #112518
Odur, thanks for the info. Your link also reports it happening with nvidia drivers as well. There seem to be at least 2 bugs with similar profiles here, possibly more. One is fixed by disabling SyncToVblank in compiz (causes hard lock with white screen). Another is an invisible login prompt. I get this with OpenGL screensavers. There may be another bug as well.
4GB RAM w/ bad BIOS causes problems
IRQ settings causes problems
disabling APIC causes problems
compiz w/ SyncToVblank causes problems
fglrx doesn't seem to work right with some newer cards
xorg may have its own issues
milanvora, is the whole system locked (not pingable) or just the console? (display, keyboard, mouse) Do you get white screen or black?
I had no problems until I switched my (very) old Matrox card for an ATI 3450 6 months ago. Since using fglrx I can't switch user. I can switch user if I use radeonhd driver, but then I can't watch video due to lack of driver support.
Therefore the problem is clearly in fglrx.
Perhaps there is a config change that can be done to fix this? I'm using default config.
However, this issue has been mentioned repeatedly over the last 6 months (at least), and the ATI devs response was along the lines of "switch user? who uses that?", so don't hold your breath waiting for a fix because they don't really seem to care.
Give it long enough and radeonhd will be usable. Of course, your video card might be obsolete by then...
I am running 64bit Ubuntu Hardy 8.04 LTS. I too have the problem of screen freezing and system becoming unresponsive on switch user in Gnome with the fglrx driver. Works fine with radeonhd, but then I get no 3d acceleration on my 4670 card!
To AMD/ATI: When will this problem be fixed? I see it still in the latest Catalyst 8.12