Yeah, I decided to make it case insensitive to avoid confusion. It seems most non-techies aren't aware of the GB vs gb distinction (and not to mention the uber-geeks who insist on gibi and kibi and all that :) )
If you want gigabits, you can explicitly write "gbit":
For the curious, there's a GPL project, 'qalculate', which could be used as the basis for similar calculator services.