But the base is where you hold the phone, usually...as is the back. In theory, i suppose, this keeps the antennae away from your head when talking....more like down by your jaw or chin (depending on the head size).
The effect on reception is negligible unless you're already in an area with very marginal reception.
CyanogenMod v5.0.6 for the Nexus One allows the screen/display content to rotate through 360 degrees....so you can hold the phone upside down with the base at the top...and happily use the screen. Yes, the buttons to touch are now at the top, but the phone isn't huge, so there is no real inconvenience in this.
But I've just found that in a location where I get slow "G" throughput, holding the phone upside down as described actually allowed me to get "H"...and much faster through put.
I'll have to play with it some more, but post this here in case anyone finds it useful.