OK, so maybe I'm an outlier on this "URL" thing. I tended to write/say "a URL" on the grounds I was articulating the letters U - R - L, instead of pronouncing it to rhyme with 'earl'. But the more I think about it, I guess the convention is to read three-letter IT acronyms as words if you can: .doc, RAM and so on, whereas the tongue-twisters have the letters pronounced; FTP, .pdf, CPU, etc.
Maybe its the fact that the vowel comes at the front in 'URL' that's thrown me.
Jeez, I hate it when I have to change my thinking. (Yeah, I really miss the Bush administration...)
cheers,
Graeme