This happened once before with a custom list I built, and it just
happened again in a document library.

I am able to use "edit in datasheet" to do mass updates, but suddenly
I lose this option- it becomes "read only".

I had no idea what happened the first time (w/ my custom list), but
just now I changed my "choice column" choices in my document library
and went into datasheet view to edit them and use the new choices. I
was able to, but after refreshing the data the view switched to "read
only".

Anyone had this happen? Any idea how to make it editable again?
Datasheet view is awesome for multiple updates, so this could be a
real problem for me if there's no fix.

FYI: The affected custom list was created in my personal site, and the
affected document library was created in a team site. It may also be a
problem in portal, but I haven't tested it.