Personally, I think the Nielsen Q2 2009 Mobile Insignts survey results as reported in Silicon Alley Insider article...

CHART OF THE DAY: What Phones Do People Dump For iPhone?

...has a bunch of problems. This is not to say that Nielsen's method is incorrect or the data is wrong. I just don't see enough in this one pie chart (almost always a bad choice for presenting data comapring more than a few items) to convince me of its validity. That said, it is a very interesting chart.

Motorola phones (presumably of a wide variety of models and platforms) were the most replaced with 29% of them iPhone 3G buyers coming from that line. "Other" came in a strong #2 with 19%. Samsung, BlackBerry, and "Older" iPhone were at 3, 4 and 5 (no percentage information provided).

It would have been a lot more interesting to me if the chart provided a breakdown by platform (feature phone, Windows Mobile, etc.) although we can somewhat infer some of this information by manufacturer. Of course, the real story from what I see is that most people are moving from feature phones (non-smartphones) like the Moto Razr and "Other" (48% -- 59% if you assume most of the Samsung switchers are not Samsung smarthphone users). With the exception of BlackBerry (#4) and 1st generation iPhone (#5), it doesn't look like most people are moving from a smartphone to the iPhone.