It's really, really not recommended, but... if you empty the table first (otherwise you will get problems when it "catches up"):
DBCC CHECKIDENT('MyTable', RESEED, 1)
MSDN:
http://technet.microsoft.com/en-us/library/ms176057.aspx[
^]
[edit]Oops! Set it to zero, not one - OriginalGriff[/edit]