Expect ML to be released in conjunction with a new MBP or iMac. I don't think Apple would release updated hardware, then, 1 month later, release a new OS. It doesn't make much sense. Its much more attractive to sell new hardware with a new OS preloaded. Sadly.
Also, note that ML is "arriving this summer." Apple doesn't say when specifically because it restricts them to a release date. This way, Apple has more leeway and can arrange a release to occur in the most sensible way, both in a business sense and a marketing sense. My guess is the new OS is finished and they are waiting on Intel so that they can release an "all new" product for sometime in summer - which happens to be so vague that it could be in early June to Late August. In consumer driven business its all about setting yourself up for a plan B and a plan C without anyone ever knowing.
Huh? Were you not here last year with the last iMac refresh in May 2011? I remember it like it was yesterday because I was in the market for the new iMac and everyone was salivating for the summer refresh. There was speculation that the new iMac wouldn't hit until OS X Lion came out (which eventually hit in July 2011) but that didn't stop them from releasing it in May. Even worse, those who bought the refresh in May weren't eligible for the free upgrade to Lion since they were two months out (those who bought it in June got the free upgrade).
Apple doesn't delay hardware based on software. It makes no sense waiting for newly released components to sit in a warehouse for two months and waste time becoming obsolete just so they can time it with an OS release.