My two cents:

Cranberries are yummy and if you like them and they help you, great! Most research has shown that European species have more anti-effective properties than american ones, and the key seems to be more in the increased fluid intake than the cranberries themselves. Stay well hydrated and flush out that bladder frequently!

Urethritis is the real discomfort you feel. Whether it's a bacterial infection, chemical irritaiton (if you use ButtPaste or another anti-chafing cream, or if your diet is heavy in caffeine or spicy foods), or direct friction, it can hurt like heck. Sounds like yours was from the bacteria. Research has NOT borne out that exterior clothing impacts UTIs (but it really does for yeast infections). That being said, using proper riding shorts with a chamois with antibacterial properties can't hurt and certainly might help. The padding from the chamois may also help prevent direct trauma to the spot that hurts (and may be allowing normal local bugs in where they shouldn't be). Using a seat with good anatomy for your body may also help.

You may fall into the category of women who simply develop UTIs after specific activities. The classic one is women who get them after having sex. Prophylaxis in the form of one antibiotic pill after sex has worked wonders for many of these women. If you develop this sort of problem after every ride and have a well documented track record, finding a doc willing to help you out with a per-instance prescription for Septra may be what you need (should other more conservative measures fail).

I hope you find a good solution and keep riding!

--Lisa (who, despite actually being an MD, doesn't intend for this post to constitute direct medical advice -- because you're not my patient, I'm not your doc, but I'd like to help out other biking women as much as this media allows! )