I haven't had a chance to review the 680, but have discussed the differences with Palm. The biggest differences are:
The 680 uses a new Phone application that combines favorites, dialer, wallapaper, recent items into a very slick interface.
The 700p supports EVDO for broadbandlike speeds whereas the 680 only supports EDGE.
The 680 will be cheaper, but no word on pricing yet.
The 680 is available as an "unlocked" version allowing you to use on any GSM network
The 680 is smaller and lighter (smaller battery, slimmer case and no antenna)
Both run Palm OS
They both use the same cables/chargers
The 680 will come bundled with 1 month free of Yahoo music, stereo headset. (not sure how long this promo will last, but will be available at launch)
The 700p has more user memory
Both use SD cards
Both support Bluetooth 1.2
Top of my head, those are the distinctions between the two.
If you have further questions, don't hesitate to ask in the
Treo 680 forum.