What is a good way to display infinite nested comments?

Is there a way to do infinite nested comment threads in a way that doesn't look horrible? I realize that one could simply restrict the levels of nesting, but perhaps there's a better way of indicating some comments are replies to other comments than simply indenting the comments. After a certain level of nesting, indenting fails.

Some ideas to play with: color, size, progressive disclosure, numbers (1.1, 1.2, 1.2.1, etc), modal windows, a comment reply stream (like Facebook or Twitter), something like SeaDragon, etc.

Has anyone solved this problem? What did you do? Out-of-the-box suggestions and visual examples are welcome too.