When you record HD Video, you need to be able to buffer possibly a minute (or more) of -raw- video. That takes a large amount of RAM. You need to queue the video for compression without dropping any frames. When you consider there are background operations going on (like email polling, cell tower negotiations, background apps, etc) as well as the write time for solid state memory... It wouldn't surprise me to see 60MB-100MB needed just for Record mode.
Now, I expect the video editing app also needs to cache clips and such, especially when doing the OpenGL 3d transitions. Keeping two 15 second HD clips (uncompressed) in memory to do cross fades or 3d rotate/flip operations takes loads of RAM.
Playback is trivial, compared to recording and editing with transitions.
Before the HD Video Camera, you had apps that had a couple dozen HD sized images. With Video recording and editing -- you now have apps with thousands of HD sized images all getting shuffled around in real time with no pauses or jitter.
That said, I think Video has helped raise the bar on iOS memory, and we'll see more 512M+ devices in the future.