OK, from a marketing standpoint I can't believe that you would have to continue to buy more pedometers - I don't know how you would change/charge a battery, assuming there is one (could use the compression from your foot for the limitedpower it needs - see ecotype watches) but there must be something
My problems with it:
1) It says you can leave the pedometer in when your not using it - which is good cause it looks like you'll have a hole in the bottom of your shoe otherwise (hopefully they will be smart enough to have an insert I'll lose after a month)
2) Most armband holders I have seen cannot accept the additional piece that you need to attach to the bottom of the Nano and still work (see Apples loose fitting one for example, and mine has a velcro strap that uses that spot to hold the nano in place).
Good idea: I'll wait a few more months for somebody to copy it with a pedometer that doesn't require a $100 (msrp on the website) shoe with a hole in the bottom.