I haven't seen anyone talk about this..

Does anyone just have alternate wheels and chains, so they run fixed at times and geared at others?

Seems logical to me, so is what I am doing for a couple reasons:

1) I have too many bikes already - dont want any more cluter

2) I *really* like my Mondonico - it fits perfect and has great geometry.

With 2 chains with SRAM "Power links" and quick release, I can switch in 5 minutes...
The extra 2 pounds (at most) of the derailleurs, extra chain ring, etc doesn't bother me a bit.

Any others do this?

No, but don't let that stop you.

switching would be a lot easier if you had internal gearing on your geared wheel, no switching chains, just bolt it on, mount the shifter on your bike and your ready to go. And with a 7 or 8 speed internal hub you've got a decent range of gear ratios, if you really want to amp it up there's a 14 speed hub avalible, but it's not cheap.

I did this during cross season last year. It works fine... I only had a single ring up front though and used a mtb cassette in the back.

this would be harder since you couldn't just leave the shifter and derailer hooked up all the time.

Ditto running single up front with der & shifters (bar end) always hooked up. This was on my '80s Schwinn commuter.

With the wheel I have, the small ring (39) lines up perfect, so it works great