Can I automatically log in to open WiFi that requires web login/password?
The best you can do is set it up to auto-connect to the network and then manually open a browser when you're in range.  Save the username and password in the browser.  That gets it down to 2 clicks (er... touches).  Make sure Remember Passwords is on the the browser's settings.

There are some apps that claim to do what you want but I've never used them.  Here is a list.