I know this forum is more about the extremes of light bikes, but I'm looking for an urban bike for San Francisco, preferably with an internal hub. Do they get much lighter than 20lbs? The lightest I found was the Sexy Bicycles Mercury.

Are you looking for off the shelf only, or willing to build your own for less than $2500? (Which is possible, and will be lighter)

_________________
Exp001

Willing to build.

For commuter type bikes the Spot Acme is pretty light due to it's Al Frame.

Raleigh is making a pretty sweet belt driven 29er with an alfine hub... well under your price ceiling

Hmm...couldn't find it. And wouldn't that be heavy?

A company called Dynamic.
$999 = 8speed internal hub, road frame (al), drop bars & shifters. Upgrading to an Alfine 11 would be cheap, still putting you well under the $2500 price call, and the 11 is slightly lighter apparently. And it's black.
Reviews around 'the web' look good. Google-fu that sht.

And with the extra money you can build the wheels custom with some 340 rims, a super light front hub, and perhaps even upgrade parts to lose more weight (fork, handlebars, seatpost, etc:.) ... you could probably go "as light as possible without being fully custom" with this one. Once you've got the basic setup, you can go about lightening it up by replacing various components per your budget. Ain't nothing stopping you from running iLinks on an IGH for example, or getting light-weight brakes, etc:.

_________________
Exp001

Nice! Kind of ugly, and I hate under-top-tube cable routing, but that's probably the lightest IGH bike around.

They actually do have the 11-speed version too. But odd that the website isn't updated. Best is to email them on that if 11 speed is preferred.