Google's working on an update to its Gmail app for Android, adding a floating header/reply bar for usability and some real overall speed improvements. A beta of that better version is in the wild, and phones running Froyo (2.2) can try it out.
The Phandroid blog offers up a multi-site download link, drawn from the good folks at the XDA Developers' forum. You don't need root access to your phone, but you will have to head into Settings, open the Applications section and enable installation from unknown sources. As far as getting the .apk file onto your phone for installation, you might consider using Dropbox or transferring it to your phone through a USB cable.
The APK update won't work on every phone it seems. If you ever want to head back to your phone's stock Gmail app, head to the Applications menu, then select Gmail in the All tab. Press the "Remove Updates" button and you're back to where you were — though we find very little not to love in this update.
New GMail Now in .APK Form for Anyone (on Froyo) to Install [Android Phone Fans]