Can I Update my T-Mobile G1/HTC Dream so it runs Android 2 Can I Update my T-Mobile G1 (Also known as the HTC Dream) so it runs Android 2?  Is it possible to do this without rooting the phone?
No.  You must root your phone and install a mod such as CyanogenMod to get your phone running a semi-equivalent of Android 2.1+.  There is no stock rom for Android 2.1+ for the G1.  Additionally, if you do go with CyanogenMod, not all features are fully supported due to hardware limitations, such as LiveWallpapers.