mk. i have this one:
http://www.belkin.com/IWCatProductPage.process?Product_Id=277970
it actually used to be sold through the apple store. (i got it before the time of the iphone). although this version does not appear to be compatible with the iphone, look for these types of devices. they are small and very convenient.
i would NOT rely on your iphone voice memo. i've tried on several occasions when i neglected to charge my ipod battery... the iphone 3g (maybe they've fixed it if you have more recent hardware) only records for ~30 min and then shuts itself off without warning. so as you're thinking oh, ill just go back to that... you're actually screwed because it wasn't recorded. ohh good times lol.