There has been a lot of debates whether Linus will release the next major Kernel as 2.6.40, 2.8, or 3.0. Well, the answer is now definite. Linus will use a new naming as Linux Kernel 3.0 for the next release as he already tagged 3.0-rc1. The reason is because the number in the current 2.6.x series has reached 39 and the development has grown so much. In fact, previous 2.2 and 2.4 didn't get this big, so he thought it would be nice to bump the kernel version to a fresh one and thus it goes to 3.0.
So, the next Slackware release will likely use Linux Kernel 3.0 for it's default kernel. It has big improvements to previous kernel releases, but there might be some drawbacks. The 3.0 kernel will likely to remove some old hardware supports, so if you are using an old hardware, this is probably the perfect timing to invest for new hardware in the future.
If Slackware keeps using SLACKWARE_VERSION.KERNEL_VERSION naming scheme, then the next Slackware release could be 14.30 14.0 (Thanks to Pat for correcting this)