Zune HD users should be able to download the latest firmware update now, and it's substantial enough to warrant grabbing ASAP. There's a speedier browser, predictive text entry (a la iPhone), and that's just the beginning. Updated:

Update: The firmware upgrade went through just fine on my 32GB Zune HD, and I'm definitely seeing an improvement in the browser's speed. Before, it was mysteriously slow, especially compared to the iPod Touch's browser—there's no reason for that, especially given the Zune HD's speedy Tegra processor. Even with the upgrade, it's slightly slower than Mobile Safari, especially when loading heavy sites like Gizmodo, but it's noticeably faster and not at all irritating anymore.

It's good to see Microsoft offering optimizations, but we're still waiting for a more major firmware upgrade: Social networking, IM, and YouTube should be top priorities. That being said, I still love the Zune HD—it's even wowed the notoriously hard-to-please Brian Lam.

Browser:

•Faster performance in navigating web pages
•Option to render pages as Desktop or Mobile (Option in settings)
•Better Tap-To-Zoom
•Better ability to scroll when typing a message online

Keyboard:

•Predictive Text
•Auto capitilization
•Dedicated Settings Menu
•Wider keyboard in landscape mode

Music:

•Artist bios now contain links to songs, other artists, and other albums
Radio:

•Choice of seek method (presets or seek) in settings
Other

•Apps load slightly faster, and the accuracy of the touchscreen is improved during game play.
•Apps close faster
•The power off says "slide down to power off" instead of just "Power Off"
•Faster Startup (less than 15s)

The download should be available through the Zune software, but some 16GB users are reporting it hasn't been pushed through to them yet. If you can get it, report back with anything else new/better you find. [GDGT, MobileTechWorld, thanks Ubi79!]