With iPad pre-orders having begun in the U.S. just a few hours ago, observers are looking for any signs that might indicate how sales are going. According to one report , two orders placed 30 minutes apart this morning resulted in Order ID numbers approximately 10,000 apart, suggesting a rate somewhere near 20,000 orders per hour if order numbers are issued sequentially as they appear to be.

Now, of course, we can't be sure every order was for an iPad. Apple does sell other stuff. But at 830am in the morning on the east coast, my guess is that most of the orders were for iPads.

Fortune has followed up with a report on a more organized effort that looks to be showing in excess of 50,000 orders in two hours, roughly in line with the earlier estimate.

"51,000 orders in two hours," announced Victor Castroll shortly after noon. He's an analyst with Valcent Financial Group and an AAPL Sanity member who, with the blogger-analyst who calls himself deagol, has been monitoring the spreadsheet.

The estimates of course come with several caveats. On the downside as mentioned, not all of the orders being placed through Apple's online store are for iPads, although it is likely that a very substantial portion of them are, given the excitement over Apple's tablet device and previous launches.

But on the upside, the data includes order numbers, not unit numbers, and it seems reasonable that some purchasers are ordering multiple iPads at the same time. Apple has restricted customers to two iPads per person, limiting but certainly not eliminating this effect on the estimates, suggesting that 50,000 iPads in two hours may at least be in the ballpark.