I've had a lot of problems with Yahoo not having complete historical data, especially on thinly traded or OTC stocks. I wouldn't recommend using their data in a production application.
yeah that caught us out - we found that even though the data is freely available from those sources, you're not allowed to include that data in a commercial app :-(
Oh really? That's interesting - does that apply even if it's not distributed with the commercial app? I'm sure I've used trading platforms before which use Yahoo...
Our understanding of the Yahoo/google T&Cs was that we couldn't aim to profit from their data (http://finance.yahoo.com/badges/tos). We may be being overcautious but we didn't want to risk getting into trouble ;-)