I would just be tickled to death if we were to ban tab characters from asdf code (I'm convinced by jwz's argument, and I've been harmed enough times by Windows-versus-others tab issues), but I'd be ok with keeping them.
Do we have a consensus on tabs-versus-spaces?
For general prefix indentation purposes, I prefer tabs. Something about the semantics of "indent" versus the fixed-width-font-specific "4 or 8 spaces" allows the end viewer to easily change indentation to fit their display device. The normative indentation rules for C/Java languages support tabs rather nicely (with few cases where spaces would be preferred).
Expecting tabs to line up with other characters is evil. (corollary) Mixing tabs and spaces is evil.
Lisp code is traditionally indented at non-even spacings, with the explicit expectation of alignment. Thus I think spaces-only is the current best practice for lisp. In an ideal world, the editor would take care of all display formatting, and whitespace would only be used for tokenization...
- Daniel