You can enter text into Basecamp discussions, to-dos, and text documents. And you can use basic formatting like bold, italics, lists, and quotes.

However, I once copy/pasted text from another program and saw an inline hyperlink appear ("some text" anchored to a web address). I really want to use inline links, but I can't figure out what markup is required.
Wikipedia notes 37signals as Textile users. And there's an old "product" blog post that indicates that it is Textile. But, the linked reference page is now defunct, appearing to be a cached version.
I tried the basic Textile link syntax and a variety of Markdown formats, but nothing worked.
"some text":http://example.com