Ted Ts'o has mentioned on his blog that Android Gingerbread is using EXT4. Of course, it's also up to the phone vendor as simply flashing your firmware won't reformat your disk storage with a new file-system. One of the first phones to use EXT4 with Android 2.3 is the new Google Nexus S smart-phone.
Currently most Google Android-powered phones are using the YAFFS file-system, but there are a few devices out there already opting to use other file-systems. Though with the move from Yet Another Flash File System to EXT4, some developers may need to modify their Android applications for better performance and data integrity, as is mentioned on the Android developers blog by Google's Tim Bray. "[EXT4] buffers much more aggressively; thus you need to be more assertive about making sure your data gets to permanent storage when you want it to."
Other highlights for the just-released Android 2.3 include user-interface improvements, more intuitive text input, improved power management, greater control over applications, SIP Internet calling, downloads management, and near-field communications support. Enhancing the Android platform for developers with Gingerbread is a concurrent garbage collector, faster event distribution, and updated OpenGL ES graphics drivers. There's also new Android APIs for gyroscopes and other sensors as well as support for reading these native input events. These new features are talked about on the Android 2.3 Platform Highlights page.