Why does the web browser on my iPhone 3gs (3.1.3) keep forgetting my login names/passwords for various web sites (https, basic auth), and how do I fix it?

It seems to remember them for a while, but then it eventually forgets them. After a few days, maybe? It's hard to tell, but it sucks. I want it to remember them forever and always, just like Safari on my desktop does.

Settings / Safari: AutoFill = On, Names & Passwords = On.