<input>
without a <form>
appears valid, yes (at least for html 4.01, look near the end of 17.2.1):
The elements used to create controls
generally appear inside a FORM
element, but may also appear outside
of a FORM element declaration when
they are used to build user
interfaces. This is discussed in the
section on intrinsic events. Note that
controls outside a form cannot be
successful controls.