Two letter domain names TLDs are explicitly not shortened into t.co links by twitter-text. Examples are a.co, bit.ly, c.us. Either prepending an http:// or appending a / will turn those into entities. Three letter TLDs always seem to be converted into entities.
This seems to be controlled by the Twitter::Regex[:invalid_short_domain] regex in twitter-text. I would expect that either none of these style "URLs" would get converted over to entities or they all should (with the exception of 't.co').
Understood if this was a pragmatic decision based on likely hood of 2 vs 3 letter TLDs being typed in without the http, but maybe it should be documented to avoid future confusion. Also understood if you don't want to bother because I'm the only one anal enough to check.
Updates
- 2012-04-11 Issue filed with team. Thanks.