Difference between the Sennheiser i300 and MM70i headphones for iPhone/iPod/iPad
Huh! So I asked an Apple representative, but they didn't know. I also asked Sennheiser and they finally got back to me. Apparently the i300 are exclusive to Apple (as I suspected), and although they look very similar to the MM70i, they're apparently closer in sound performance to the cheaper MM30i.

So basically, the Apple exclusive Sennheiser i300 are equivalent to the Sennheiser MM30i.

You can see the MM30i here:
http://www.sennheiser.co.uk/uk/home_en.nsf/root/private_headsets_mobile_music_504739   

According to the Sennheiser representative, the MM70i has a better bass response and more accessories than either the MM30i or the i300.

So, from what I can gather, it seems buying the cheaper MM30i, or the more expensive MM70i, is a better option than the Apple exclusive i300. 

Update: Since I wrote this question, it appears that Apple have reduced the RRP of their i300 to match that of the MM30i. However, as Amazon seems to consistently offer a 30% discount on the MM30i's, the Apple exclusive headphones are still more expensive in a practical sense.