Are retina graphics a burden all Macs (including non-retina) must carry? I'm in the process of downloading a rather chunky iMovie update (1.89 GB), and I'm wondering if the "Retina support" described about the update is something my non-retina MacBook must have lingering around?

I, like pretty much every other person, have a non-retina Mac and are these unused retina graphics are going to be present as inactive redundancies of all interface files/graphics in all retina supporting apps?
Yes, retina optimized apps will typically ship with both versions of graphics.

You can easily have a look at the retina graphic files yourself by viewing the package contents of the app. They should be located in AppName.app/Contents/Resources. At least for the Apple ones, the retina versions seem to have '@2x' appended to their base name, for example badge-info-N@2x.tiff is the retina version of badge-info-N.tiff.

While you probably could delete these files, it probably isn't a good idea as it will break the code signing, which in it's turns makes KeyChain access impossible without signing the code again, for example.

As to how much overhead or 'burden' the retina graphics are, I had a quick look at iPhoto.app (I don't have iMovie installed). In total, the retina graphics seems to add about 10% to the total app size (or in iPhoto's case, about 124MB). I think for a typical user, when most apps are retina-optimized (not sure when/if that will happen), this will not amount to more than 1GB in total.