Assuming Apple (AAPL) reports record third quarter revenue and earnings  this afternoon — the Street is looking for year-over-year increases of 51% and 54%, respectively, according to Thomson Financial — what else is there to say?

Like Kaufman Bros.’s Shaw Wu, who issued yet another note to clients this morning, we’re expecting Apple to once again beat expectations, although not necessarily as handily as it has in recent quarters.

Here are the numbers we’ll be watching:

• Mac unit sales: The Street is looking for Apple to report unit sales between 3.1 and 3.2 million Macs, depending whose consensus numbers you believe. Most of those estimates, however, were filed before Gartner reported quarterly domestic Mac sales up 24.7% year over year and before the NPD Group reported U.S. retail sales up 11% in June (following increases of 39% in April and 35% in May). Based on those data points, Bullish Cross‘ Andy Zaky raised his Mac estimates overnight from 3.1 million to 3.36 million.
• iPad profit margins: The sharp drop in Apple shares over the past few days was attributed, in part, to concern over the adverse effect of blow-out iPad sales on the company’s profit margins. The more iPads Apple sells, by this line of thinking, the worse Apple’s margins. But nobody outside of Cupertino really knows how many low-margin $499 iPads vs. high-margin $829 iPads it sold last quarter. We could get an upside surprise this afternoon.
• iPhone channel inventory: Financial Alchemist‘s Turley Muller, the blogger with the best track record of any Apple analyst we track, surprised a lot of investors when he estimated Q3 iPhone sales of 7.8 million  — well below Street-high estimates in the 10 to 11 million range. The problem, as he sees it, is not iPhone sales — they’re going like gangbusters — but iPhone shipments. The company started the quarter with 2.7 million iPhones in inventory and ended the quarter with roughly zero. That means 2.7 million fewer iPhones than usual left the assembly plants in April, May and June. Could be a downside surprise.
• Gross margin: Some analysts believe that the gross margin ratio — revenue minus cost of goods divided by revenue — is the most important number of all, because it tells them how efficiently Apple is turning sales into profits. Three months ago, Apple warned investors that third-quarter margins might be as low as 36% — far below the 41.7% the company reported for Q2. The Street’s looking for 39%. Anything higher could be a big plus.
• September guidance: This is the number that savvy investors have learned to discount but which can still send the stock spiraling down in after-hours trading. Apple traditionally guides “conservatively” — which is to say it low-balls its forward-looking estimates so it can handily beat them three months later. Take, for example, gross margin. In a note to clients issued Monday, Piper Jaffray’s Gene Munster pointed out that over the past 14 quarters, Apple has beaten its own gross-margin guidance by 293 basis points — which may be where the Street is getting its 39% consensus. According to chart maintained by AAPLinvestors, the company has beaten its own revenue guidance 21 of the past 22 quarters. Average beat: 11.3%.

Bottom line: If CFO Peter Oppenheimer warns during the earnings call this afternoon that Apple is going to be hurt in September by bad foreign exchange rates, rising inventory costs and back-to-school promotions, take it with a grain of salt.

Apple usually reports its earnings about a half-hour after the markets close. A conference call with analysts is scheduled to begin at 5 p.m. EDT (2 p.m. PDT) Tuesday and should last about an hour. Tune in here for our analysis.

See also:

• The Street awaits Apple’s Q3 earnings

[Follow Philip Elmer-DeWitt on Twitter @philiped]