What is the average upstream capability (in kilobits per second) of a
residential broadband connection (cable, dsl, etc)?

I don't know how you'd find an overall average, or whether it would be
useful for anything, since there are so many variables. One Web site
collects and displays broadband speed report data here:
  http://www.dslreports.com/archive

Thanks, but what I was looking for was a nice overall average.  On the
whole, do houses in the US have 800kilobits upstream?  20kilobits
upstream?

15-20 kilobytes a second.  That's average for cable and most dsl. 
Some DSL providers might give you up to 40 k/s though.  You're not
going to get 800 kilobits/second unless you're on a T1 or something
though (which you can actually get for quite a hefty cost).  Although,
if you're on a T1 you'll get faster than that probably