Is there anyplace I can get data about temperatures in a certain city
or region, over a number of days?

Actual temperature, or Average temperature is fine.

I am trying to plot the temperature of Boston against the temperature
of Tampa, to explain that it's not THAT cold up here.

Hello E1ven,

Weather Underground provides current and past weather data.
Enter City and State of your interest for current weather
http://www.wunderground.com/

Past and custom dates are also available.

For example:

Weather for Boston, Massachusetts from November 16, 2004 to November 15, 2005. 

This table provides the high, low and average temperature for each day. 
Wind, humidity and rainfall data is also provided.
http://english.wunderground.com/history/airport/KBOS/2004/11/16/CustomHistory.html?dayend=15&monthend=11&yearend=2005&req_city=NA&req_state=NA&req_statename=NA


Weather for Tampa, Florida from November 16, 2004 to November 15, 2005. 

This table provides the high, low and average temperature for each day. 
Wind, humidity and rainfall data is also provided.
http://english.wunderground.com/history/airport/KTPA/2004/11/16/CustomHistory.html?dayend=15&monthend=11&yearend=2005&req_city=NA&req_state=NA&req_statename=NA


Search terms used: 
Weather Underground


I hope the information provided is helpful!

Best regards,
Bobbie7

That was great, thanks!

Thank you for the five stars and tip!
--Bobbie7