As long as the Lightroom/Camera Raw pipeline is matched, Lightroom does not automatically create a saved version until you actually save the file (I just double checked). If your pipeline is mismatched (such as LR 3.3 and ACR 6.2) and you get a warming asking about rendering the file via Lightroom, it goes back to the old behavior of auto-saving the file before editing in Photoshop.
BTW, are you using Lightroom 2 with Photoshop CS5? That would be a mismatch...LR 2 and ACR 5.7 in CS4 should also not auto save...LR 2 and CS5, I don't know, never tested that.