The kernel was released in its newest version, 2.6.21. This version still doesn’t feature the new WLAN stack, but many distributions include it anyway.
As usual with kernel releases you can get a short but informative summary about the changes at kernelnewbies.org. There you will find the most interesting changes (aka “the cool stuff”) as well as the lesser interesting but still interesting enough ones (like new drivers, etc.).
Some of the cool stuff include VMI, the Virtual Machine Interface which aims at becoming the common paravirtualization interface for hypervisors, dynticks and a new “Dynamic kernel command-line” – and of course various other things.
However, the new kernel does not include the new WLAN subsystem. Sad, but not too sad since distributions like Ubuntu and Fedora include it anyway (already mentioned here). And when it is included in these distributions I doubt that the mainstream kernel will wait too long till it also includes the new system.