Wordpress and 301 redirects - why does it break?

OK, so I've noticed this on a few of my Wordpress blogs and never bothered to look into it, so I figured I'd ask if anyone here has any thoughts on this topic.

Each time I try to use a 301 redirect to flip one of my non-www domains to the www version on a Wordpress site, I get an error.

Any ideas why or how to get around this?

I know I can spec which domain Google should look at in WMC, but I like my 301s - they're so artful and fancy.

There are a number of conflicting things in Wordpress, and there are also a lot of Duplicate Content gotchas too.

I have seen a detailed tutorial or thread about fixing Wordpress problems somewhere else, sometime last year I think.

dupe content stuff I can lick in a couple of ways - it's the damn canonicalization thing that's vexing me...