There is inconsistency in how various operating systems display file size units. Windows shows base 2, MacOSX shows base 10. Does Ubuntu have guidelines for this?
Ubuntu does have a policy for this. The basic gist is as follows:
Use base-10 for network bandwidth (e.g. 6 Mbit/s or 50 kB/s) and disk sizes (e.g. 500 GB hard drive or 4.7 GB DVD). When using base-10, follow the SI standard for prefixes (e.g. kB, MB).
Use base-2 for RAM sizes (e.g. 2 GiB RAM). When using base-2, follow the IEC standard for prefixes (e.g. KiB, MiB).
There is the library libkibi, available starting with Ubuntu 11.04, that provides helper functions for formatting sizes appropriately.