[Hexstream]

I find your lack of parentheses disturbing. I'd personally prefer:

  (b-when foo (plusp 1)
    (frob foo))

These implementations also don't handle declarations correctly ;P A surprisingly tricky topic.

[felideon]

Hmm, did you mean...

  (bwhen (foo (plusp 1))
      (frob foo))

bwhen is actually the first version of the macro, heh. It does seem a bit more idiomatic with parens, but I've gotten used to the other.