According to a report, the problem occurs because the 32-bit call emulation layer does not check whether the call is truly in the Syscall table. Ben Hawkes, who discovered the problem, says the vulnerability can be exploited to execute arbitrary code with kernel rights. An exploit (direct download of source code) is already in circulation; in a test conducted by The H's associates at heise Security on 64-bit Ubuntu 10.04, it opened a shell with root rights.
The kernel developers have remedied the flaw in the repository, and Linux distributors will probably soon publish new kernels to close the hole. Until then, switching off 32-bit ELF support solves the problem if you can do without this function. For instructions, see: "Workaround for Ac1db1tch3z exploit".
Hawkes says the vulnerability was discovered and remedied back in 2007, but at some point in 2008 kernel developers apparently removed the patch, reintroducing the vulnerability. The older exploit apparently only needed slight modifications to work with the new hole.