According to Google, the first change is faster and improved scrolling action when navigating through your inbox. Google says that the scrolling speed now reflects the speed with which users swipe up or down the screen. This can come in handy when you have really long email strings threaded together.
Google has also addressed the tool bar when scrolling. Several weeks ago, Google updated the tool bar, which floated down the inbox as users scrolled down. It was nice to have the tool bar always accessible, but the way it floated on top of emails was weird and just didn't look good. Google apparently agreed.
Now, the tool bar remains anchored at the top of the display as users scroll through their inbox. Having ready access to the tool bar is helpful when you want to perform actions such as replying, forwarding, starring emails and so on.
Google says that it made these changes based on user feedback. It also noted that it is welcome to further feedback and/or suggestions from end users. The changes are reflected on the iPhone and iPod Touch, but not the iPad (which has its own HTML5 version of Gmail).
After playing with the new browser-based version of Gmail, I have to say the improved scrolling and tool bar behavior are welcome updates. I especially like how fast the inbox scrolls now. (Thanks, Google!)
Google suggests that users who don't see the changes in their browser empty out the cache and try again. Right now, it only works in English.