My Oxford Names compation gives:

Wyatt - from the medieval given name Wiot, Wyot, Gyot whcih derives from the OE personal name Wigheard - hardy, brave strong, variants Whyatt, Guyat(t), Gyattm W(h)yard.

Hyatt on the other hand is Scots, from Highest, a Scots habitation name, probably from Highgate - still OE though. Although it could come from the Jewish Chait.

Pyatt, though, isn't in the Oxford book.

Rootsweb points out a mailing list that would be relevant - the Piatt mailing list.
PYATT. A mailing list for the discussion and sharing of information regarding the Pyatt surname. Anyone interested in researching this surname and its variations should subscribe to the Piatt mailing list described above which is being run to encompass all variations of the Piatt surname (e.g., Piat, Piot, Pyott, Pyeatt, Pyeatte, Piet, etc).

The list is archived on Rootsweb and I found (but haven't read) this thread discussing whether the name is English or Italian:
[archiver.rootsweb.com]

(The author assumed the name Piatt was French).

Just discovered my book has 30 blank pages where Piatt should be!

Doug

Doug Weller

Director The Hall of Ma'at
Doug's Skeptical Archaeology site::
[www.ramtops.co.uk]