The OP didn't ask how to delete contacts or history. He asked how to delete the storage space. He even detailed how one option included formatting from a desktop PC using the Mass storage USB mount. The rm command I posted does exactly the same thing only easier.

Removing the directories from MyDocs harms NOTHING. All of the directories used by internal applications like maps and pictures would be automatically regenerated the next time those applications were started and instructed to write to the path name.

Where is the FUD coming from? Screwing with the MyDocs contents will delete media, but will not harm the N900's normal operation.

Agreed that rm -Rf is not the most fool proof method of data removal, even dd is not perfect if only run once. But this isn't a CIA laptop here, it's a phone and we shouldn't scare people away from simple to perform commands that provide a commonly acceptable method of data removal just because of blanket FUD like:
"Who KNOWS what could happen!!?!?!"

Head's up: We know what would happen. The data would be removed, and the N900 would still work. Get over it.