How many over 100 degree days have been reported anywhere in the US
over the past decade?

I don't have a definitive answer, but a good ballpark in this article:
http://www.gsfc.nasa.gov/gsfc/educ/science/2000/02-11-00.htm

"The first 100 degree days usually occur in Texas or Arizona but not
until late March or early April. From late April until late September,
you can count on some place in the States having a maximum temperature
over 100. Even in late October, 100 degree temperatures may be found
in most years somewhere in the southwest U.S."

USA Today has an archive covering each day for the last 7 years (but no total):
http://www.usatoday.com/weather/wext0.htm

According to NOAA there have been as few as 3 (1991) and as many as 51
(1998) 100-degree days in a given year in the past decade or so, but
I'll leave it to a paid researcher to locate and get you the full set
of data.

I take it back!  The site I found with 100-degree day information
appears to be regional, rather than national information (though I'm
still not certain exactly what area it pertains to...they're a little
unclear about this).  Anyway, you can check it out yourself at:

http://www.srh.noaa.gov/maf/html/100Ftable3.htm