The jump to 3.x was a more or less arbitrary decision made by Linus because the 2.6 version numbers were getting very large. Switching to 3.x should basically be the same as any other kernel upgrade, possibly with some additional issues with software that assumes things about the form of the kernel version number. There shouldn't be any distros holding back from upgrading; in fact Ubuntu 12.04 uses a 3.2 kernel.
There is no 'major version jump' difference between the two. The reasoning was simply "2.40 is getting unwieldy as a number, so I'm going to start 3.0 here".
