Kernel ABI is the exported binary interface provided by the kernel, and accompanying modules. Most Linux distributions have a way of tracking this ABI to detect when the kernel's exported interfaces have changed, and thus, modules compiled for it may be in need of recompiling. Usually this results in the incrementing if an ABI number (on Ubuntu, it would be the "-2-" in 2.6.24-2-generic).
There are many extremes on how this ABI is handled, from taking expensive measures to ensure it doesn't change in a released distribution, to just not caring at all (not tracking it will be an idiotic extreme I wont even cover).
In Ubuntu's case, we fall somewhere in the middle. We track it, and bump the ABI fairly reliably if _anything_ in our kernel ABI changes. We also try not to bump it in released kernels too often (security updates being an exception), but we don't put much effort into preventing it, or guaranteeing a subset of symbols will never change over the lifetime of a product.
The general consensus has been (at least for me, I can't speak for Canonical on this point) is that the only things that cared about ABI bumps were proprietary and/or third party drivers. For open-source third-party drivers, we've recently been pushing the use of DKMS. For proprietary drivers, I've been all for letting the vendors deal with keeping up with us.
However, kABI has more ramifications than just proprietary modules. Many certifications are based on the kABI it is running under (think Oracle, VMware, etc). When we bump the kABI, we in affect force a minimal recertification. This isn't good for an enterprise Linux distribution.
So it seems that things will change. For Ubuntu to be considered in certain installations, kABI will need to have strict guidelines and criteria. We'll need to justify post-release kABI bumps and do more work to rewrite security patches to avoid changing the kABI.
A bit of preparation for this has already taken place in Intrepid's kernel (based on 2.6.26). A new kABI checker has been implemented. This checker has always been there, and it runs at build time, but has been very basic and dumb. Now, it has much finer grain checks, and will be expanded to support white/black lists in which we will only bump the kABI if it affects major parts of the kernel (no more kABI bumps for appletalk, sorry guys).
You wont notice much when this happens. You probably wouldn't have known if I hadn't told you. Rest assured that it wont adversely affect our range of hardware support. Perhaps it will mean you have to do less compiling for DKMS packages on your system :)