Corporate quarters are based on the same 3-month sets you show, but any given quarter can be any given corporations 1st quarter. Looking at some prior financial reports from Apple, it looks like their 2nd quarter ends at the end of March (so their 1st quarter is apparently Oct, Nov, Dec).
If the Tablet is to be a 2nd quarter release per Apple's quarters, it would have to be out before the end of March. Now here's the fun part: the press generally likes to quote quarters as you've surmised they should be. So, if someone is estimating when a product will launch based not on Apple quarter arrangement, but instead on general time estimates, then the 2nd quarter is Apr, May, Jun.
So the key is reading the quote carefully. If Apple says: "We will release this before the end of our 2nd quarter", that's before the end March. However, if some analyst/writer says, "I believe this product will be available in the 2nd quarter", that's probably Apr,May,Jun.