YouTube has discretely begun to increase the quality of its Flash-streamed videos, adding a new 480 x 360 version with stereo AAC audio to the standard 320 x 240, mono clip created whenever a user uploads new content.  However, the site has also begun retroactively upgrading certain videos, seemingly intending to provide better quality throughout the site.  Speculation has started that Apple could be - either directly or indirectly - responsible for the upgrade; they previously were integral to Google-owned YouTube moving to the H.264 format. 

An example of the newly encoded footage can be seen here, in the video of the skateboarding dog that Apple used in demonstrating the iPhone.  Currently the higher-quality video is not an obvious option, suggesting that Google are still experimenting with the change.