… and always sit to the host’s right? Just some thing I noticed that must have a reason.
Leno’s guests enter from the right (the viewer’s right), so it’s not an absolute.
Why do most of the others enter from the viewer’s left? Probably so that they don’t walk in front of the host on their way to the chair.
But Leno’s (and, previously Carson) guests must cross in front of him to sit down.
Conan’s guests enter from the right, don’t they?
My WAG is that it’s a function of how the studio is constructed. If the “Green Room” where guests wait before coming on is on the right side of the stage that is where they will enter. If it’s on the left they will enter that way. I don’t think there is anything symbolic or ritualistic about it.
Simply off the top of my head I can think of two long-term (but now defunct) chat shows in the UK where the reverse was true. Parkinson & Wogan. Both had guests enter from the viewer’s right and sat to the host’s left.
One reason that hasn’t been suggested might be whatever side the host thinks is their “best side”. If you’re going to spend half your professional life side on to the camera, you’re going to make sure it’s looking at your best side.
OK, I guess I was wrong about the entrance location always being to the left of teh screen. But why do the guests always sit to the right of the host?
They don’t. “In The Actor’s Studio” and that new one (David Steinberg interviews famous comedians) pop to mind.
<yakov>In Soviet Russia, host Stephen Colbert enters YOU! (from the left)</yakov>
There’s no mystery in why guests sit on the host’s right. The right side is the traditional position of honor; at formal dinners the guest of honor sits to the right of the host. The ancient favoritism for the right side crops up everywhere from the Flag Code to the Book of Revelation to the military salute and civilian handshake. Why not on talk shows?
On that crappy show with Carson Daily, the guest sits to his left.
I know two or three women who actually wouldn’t mind that.
Anyone else remember when Streisand required that she sit to the right of Rosie O’Donnell so we could see her good side?
Ridiculous.
Streisand has a good side?
“Are you reading this, Charlene?!?”
I almost inhaled a Junior Mint on that one.
I’d say sitting on the host’s right is a convention, not an absolute. As to why, I’d say it’s because that’s the way Johnny Carson did it. He established the norm for talk shows, and others followed his example. Shows that sit their guests on the left probably do it for the express purpose of being a ‘different kind of show’.
Colbert’s guests sit in a seperate ‘room’ to Stephen’s left, then he runs over to interview the guest. Stephen and the guest sit across a table from one another.
Starting with Johnny Carson, all guests had to be sure their right side was their good side, and the just let the other side go. Have you ever looked at Paris Hilton’s left side? It’s like Angela Lansbury’s. 
Jack Paar’s Tonight Show guests sat exactly the same way. He established the norm for talk shows. His guests, IIRC, came in from the viewer’s right, too, just like Carson and Leno.
Steve Allen might have done it that way first with his earlier version of the Tonight Show, but I can’t find any pictures of the way he had his guests sit.
Johnny didn’t originate the format any more than he was first to have a sidekick and a band.