If i tweet something like http://74.125.128.139/ or http://[2001:200:dff:fff1:216:3eff:feb1:44d7]/ (both valid URLs), they do not get auto-shortened with t.co and they do not appear as links on twitter.com:
https://twitter.com/drymangobird/status/326728342273417216
https://twitter.com/drymangobird/status/326727659683987456
