Also there might be a possibility that their yields got so much better that they have to brand most CPU's that pass the 3440 test as 3430 due the demand over the 3440. Just like the bigger CPU manufacturers do like Intel and AMD.

OTOH the timing is a bit suspicious TI announced 720MHz version at about the same time when the mass production of N900 and the other omap3 based phones started. It could be an indication that the yield is not very good, but they now have enough chips to select faster ones in commercially viable quantities and sell them separately. I don't have any real information on the subject, so it's just a speculation.

In a similar situation is OpenPandora's second batch, since by then the OMAP3550-720 was introduced.

not really
usually the cpu runs at 125Mhz