I really like the idea of lethality Caitlyn, it can actually be a pretty effective build against squishy teams. Also Cait has a lot of ability damage (Q, traps and ult) which lethality amplifies and crits don't.

Dark Harvest doesn't work well with it though, since the point of that build is to oneshot with Ace in the Hole and DH will not trigger. Personally I'd run Hail of Blades to compensate for having no attack speed, or Predator for the bonus damage on R.

Lethality Cait is actually the best build in ARAM, I've always wanted to try it in SR :)