The open-source driver code for Ivy Bridge has been around going back to last April, but the recent Linux kernel / Mesa / xf86-video-intel releases are what offers the best support. In testing of Ubuntu 12.04 LTS, its Ivy Bridge support was spot-on, especially for the at-launch coverage. Ubuntu 12.04 LTS provides the Linux 3.2 kernel, xf86-video-intel 2.17.0, and Mesa 8.0. Assuming you are on Mesa 8.0 and a recent Linux kernel, you should be in good shape for Ivy Bridge HD 2500/4000 support.
I've only hit a few GPU hangs during my testing thus far, but they were far from being a common occurrence and each time the driver did allow for recovering gracefully. This at-launch Gen7 hardware support is in much better shape than it was last year when Sandy Bridge was just beginning to ship and hangs were much more common along with various other issues. Just boot Ubuntu 12.04 LTS or any other recent distribution and you should be good to go.
Many articles are forthcoming looking at Ivy Bridge graphics under Linux in a variety of conditions. In this first look, the Intel Core i7 3770K at stock speeds with HD 4000 graphics was compared to the Intel Core i5 2500K with its Sandy Bridge HD 3000 graphics and the Intel Core i5 2400S with its Sandy Bridge HD 2000 graphics. The same motherboard (ECS Z77H2-A2X) was used throughout testing along with having 16GB of RAM and a 240GB OCZ Vertex 3 SSD. On the software side was Ubuntu 12.04 LTS x86_64, but to make things more interesting the Linux 3.4 development kernel was used along with Git master versions of the xf86-video-intel DDX and Mesa 8.1-devel. (Benchmarks of Ubuntu 12.04 LTS on Ivy Bridge with its stock Linux 3.2 / Mesa 8.0 packages wil be published too for reference in a future comparison.)