Woodmaster has spiral cutting heads as an option.
Supposed to produce a smoother cut but I have no experience with them.
I sure would like to own a Woodmaster planer/molder/gangsaw/sander.
All the planers I have ever used had straight blades.

Pooh Bear

Here is a link that might be useful: Woodmaster Planers

I don't have any experience with Helical heads, but I have sharpened a lot of planer knives. The idea of rotating an insert when you get a nick, rather than dealing with re-sharpening the entire knife is very appealing.

You might check over at Woodweb's solid wood machining section. I've never used a helical planer either. I've heard of two types - one with a thin, flexible blade that wraps part-way around the cutterhead, and another that uses many small insert blades like Blindstar mentioned. From what I've read, the latter type is good at minimizing tearout but is prone to leave a poorer surface finish than a straight-knife planer. I get the impression that they're better suited to heavy roughing cuts than nice finish work.