How can I use Google shortening service (http://goo.gl) in Twitter for iPhone?
You need to add a wildcard to the URL to tell the application where to insert the original URL.
Your custom url should look like this: 

http://myshortener.com/shorten?url=%@


However, it looks like this isn't as simple as this on goo.gl. I'll update when I find a way without any proxy-site or other things.