How many lines of code does the average software developer write in a day?
I'd say its not how many but how good.
It will deppend on the language you are using or interested on.
I'd say that LOC isn't in any way a good metric, but as I don't know why you want to know, reading the entry post from jeff may help:
http://www.codinghorror.com/blog/2006/07/diseconomies-of-scale-and-lines-of-code.html
"10,000 LOC system would require 13.5 staff months" A month being 20 work days, about 37 lines a day. (Industry avarage in 2006)
But
"If you're wondering how much code the average programmer produces per day, I think you might be asking the wrong question. Lines of code is certainly a key metric for determining project size, but it's also easily manipulated and misinterpreted. It should never be the only data point used to make decisions; it's just one of many signposts on the road that helps you orient your project." from jeff blog
I don't think there's a programming-wide answer that would be useful to you. If there's a specific industry and language, that would be easier to find or quantify.
If your goal is estimation, you might want to take a look at these resources:
http://en.wikipedia.org/wiki/COCOMO
http://www.stevemcconnell.com/est.htm