The first use for the logo will be on the website, probably with a white-on-black or white-on-dark-grey color scheme. I will make the style match the logo, though, so don't feel artificially constrained if you have a better idea!
Secondary uses will be on business cards, invoices, etc. Any important features of the logo should still be distinguishable at smaller sizes.
UPDATE: I would very much like to see some typography-only treatments, without separate graphics. The design choices I had to pick from didn't include many type-only options. A design that simply has an interesting "N" to start the word "Ninbitsu", or maybe some other graphic effect on a different letter, could be a winner.
UPDATE 2: See the attached images I've included. These are much better representations of the kind of logo I'd prefer. Ideally, the logo would be typography that has an interesting effect on one of the N's, such that I could pull it out and use it as a stand-alone graphic, but the primary logo would be the entire word.
UPDATE 3: Added more images that illustrate simple designs that bring some kind of pictorial representation of the name into them. I would like to see some designs like these.