Files size units: "KiB" vs "KB" vs "kB"

I'm working on a web app that will show the size of certain files, and I'm wondering if there's any consensus about how to report the units of bytes. Technically the IEC has weighed in with an official standard, but I'm not sure that's worth much.

Several options:

• "1 KB" means 1024 bytes (as Windows would report it, traditional usage)
• "1 kB" means 1000 bytes (as Mac OS would report it, IEC usage)
• "1 KiB" means 1024 bytes (unambiguous, but perhaps unfamiliar terminology)

Not a huge deal since we're only talking a 2.4% difference for files in the KB range, but for MB the difference is ~5% and for GB it's ~8%.