For those MURCers in IT, you can get the final bits for 8.1 and 2012 R2 today, for all SKUs but 8.1 Enterprise, from MSDN and TechNet.
For those running Win8 who want to upgrade to 8.1 for free, you still need to wait for general availability on Oct 18. Sorry. They want to iron out driver bugs with OEMs before general release so the launch is goes smoothly.
On a slightly related topic, the next gen Surface devices are being announced Sept 23rd. Invitations to the press went out today. I have no insider info on the hardware side, but net rumors say the RT will sport a 1080p, Tegra 4 quad-core proc, 4GB RAM, two position kickstand and mobile WAN support (including LTE, NFC, etc.).
The Pro is expected to get 7-8 hours battery life on a Haswell ULV, kickstand, 8GB RAM, and larger SSDs by default (128 and 256 are the rumored sizes). Plus the mobile WAN parts.
There are rumors of a low-cost 7"-ish tablet too. No details have emerged about this one. Just very vague rumors.