I have a fresh install of Gentoo v1.0, and followed by installing KDE3 (emerge KDE). I installed the NVidia drivers per the gentoo website instructions, and updated the XF86Config file. I noticed that when I switch from X to console (via Ctrl-Alt-F1) and then try to go back to X (via Ctrl-Alt-F7), the system freezes. I can duplicate this every time. In addition, when it's in the console, the cursor appears as two small blinking pink rectangles. I changed XF86Config to use "nv" instead of "nvidia" and tried to switch to console from X and back to X again, and had no problems. I also run Redhat 7.2 with the same NVidia drivers, and I can switch back and forth from X to console with no problems, so I'm assuming this is an ebuild bug. I also checked /var/log/XFree86.0.log and noticed one error: (EE) NVIDIA(0): Failed to load glX [notice the capital "X"] So I double-checked my XF86Config file, and it's set to load "glx". Is this error related to the crash?
Don't think so. Are you using the framebuffer console? If so, turn it off. It'll cause problems. If you still have problems with fbcon turned off, then this is really an nvidia driver issue itself. You may want to ask in #nvidia on irc.openprojects.net. Someone there should have some good ideas on how to address the problem. Forwarding to azarah to comment on the glX issue.
If you are using a framebuffer console make sure that you only select the VESA option during the kernel configuration. There is also an NVidia framebuffer option and with this compiled into the kernel I was having the same problems as you, after switching to the VESA only option I now have a frame buffer console that I run at the same resolution as my X sessions (1280x1024, vga=0x31A) and no corruption. See http://www.linuxdoc.org/HOWTO/Framebuffer-HOWTO-5.html for more framebuffer information. There is are also new ebuilds for X, nvidia-kernel & nvidia-glx with a new script that helps switch between nvidia-gl drivers and opensource ones -- see the gentoo-users list for more info.
To illucidate this issue slightly, the nvidia X-windows accelerated driver is a CLOSED source driver released by the good folks at NVidia, and it is VERY VERY intense about it's usage of memory, I have found it to be a great test of both system boards and memory stability, due to this however, it tends to overwrite the memory used by the opensource RIVAFB framebuffer device in the linux kernel, and due to it's source being closed the kernel or X-Windows developers cannot develop workarounds or swap out methods to play nicely with it. Like someone said use video=vesa:mtrr in your kernel boot options, you will have a fast frame buffer that doesn't touch any NVidia reserved memory and therefore doesn't corrupt alloveritself.
I am going to mark this one as invalid, as it is an kernel/nvidia driver issue, and the nvidia fb drivers do not realy seem to be updated. Reopen if there is a kernel patch for this.