Yep, they've not only been seen, they are the wave of the future from AMD.
Check out overclockers.com if you have any doubts (or our General or Overclocking Forum here).
The new Durons and T-birds are suck (for overclocking). Better stock up now on the older parts.
A Duron 700 and a Celeron at 850 have similar performance (if you can't o/c the Duron - but you should get 770Mhz from a FSB o/c). The Duron would be more logical for a new system (although MBs are expensive)- the Celeron for a BX upgrade.