Is it required for Skyrim, if so does anyone know of a bsa version?

So far, it seems as though it's not necessary.

Although Nexus Mod Manager had a toggle archiveInvalidation option, but I've no idea what it did.

Presumably the "load loose files" check box does it for us - although no matter how many times I check it, when I open the window again it's not checked

I'm not sure what the "load loose files" option does per se, but it's not intended to be used all the time. It's something that you enable for a single session for... some reason or another.

I've never touched it, nor have I touched NMM's archiveinvalidation utility, and I've had no problems at all with texture replacers.

That's nice to know. I've never had any problems with replacers showing up either. Interesting to see what will happen when patch 1 comes out tho.

It may not be necessary but it's hard to say. I haven't any issues with texture replacers so far. However, at the same time, I'm remembering when I first started to mod oblivion. It almost always loaded the new textures before I even knew what archive invalidation was and before I started using it. It wasn't until it didn't load them that I found out about it.