Apple critics like to poke fun at the stereotype of suckers gleefully paying for an annual upgrade, but it appears that might not be the case with the iPad 2.

According to one piece of — admittedly very rough — research, 70% of people picking up the new model did not already own an iPad. That suggests two main possibilities, which aren’t mutually exclusive: some people may have been attracted by the good reviews of the original (or simply word of mouth) but intentionally decided to wait for the new version; and existing owners may have considered it simply wasn’t worth paying all over again for some relatively minor changes.

It’s important to note that the research, carried out by Gene Munster of Piper Jaffray simply involved surveying 236 people waiting in line on the first day of sales at Apple stores in New York and Minneapolis. As well as being a small sample group, that also means online buyers aren’t factored in. That wouldn’t necessarily make a major difference, but its possible that people with an original iPad wouldn’t be as likely to be so desperate to get the new model in their hands at the earliest possible moment.

Among some other results of the survey: 65% were iPhone users, 47% opted for the more expensive 3G-enabled model, and the proportions opting for the 32GB and 64GB models rather than the 16GB version were both up compared with the original iPad launch.

Munster also noted that every store he called had sold out over the weekend, and that he estimated sales in the half a million range. That’s major pinch of salt territory though: in the past, Munster’s estimates have involved a spectacular level of extrapolation, and sellouts may tell us more about Apple’s stock level decisions than the total demand for the product.