Placeholder while porting theme

Profuse apologies if this is a rather dumb question.

Is there a graceful way of redirecting visitors or simply putting a placeholder on my WordPress (2.5) site, whilst I install / test / tweak / configure a new theme ?

(I’ve tested the theme on a subdomain with imported posts and pages but its going to take several hours of testing to check everything is kosher once its ported to the live site.)