I've applied to clinical PhD programs for fall 2010, but last year, I pondered this same question myself. As a non-psychology major out of school for several years, I've only been taking psych classes part time the last year and a half in order to catch up on my pre-reqs. With less than one year of research experience, I wondered if the Master's would be a good way to get a thesis under my belt and prove that I still had the academic chops to hack it in a PhD program after my hiatus.
I took this question to several of my psych professors. The answer I got was always the same: their brows would knit, and a look of disapproval would play briefly across their faces. It doesn't hurt your chances, they'd say, but it doesn't give you any advantage, plus for most PhDs you won't be able to transfer the credits from your master's to your doctoral program. Therefore, they tended to see it as a 2 year delay, and were appropriately discouraging of the idea. The bottom line, they said, was that the great majority of acceptances are offered to students who only have their BA/BS. Get more research experience, but don't bother w/ a master's.
So I took their advice, and have ended up taking 7 undergrad classes total in time for application deadlines, (including RA for credit). I've got 3 interview invites and counting, so I guess I'd say they were spot on. Time will tell...but that's was I was told. Does that jive with what you've heard?