Note that while a capital 'K' is necessary for wiki linking, the correct SI prefix is a lower-case 'k'. Gads it annoys me when applications use "KB" as an abbreviation for kilobyte. It's "kB" please!
If kB was correct, I'm sure Apple would know that too and use it in their apps, but they don't. Look in the Finder, they use KB. I"ve always seen it as KB not as kB. KB = kilobytes, kb = kilobits.
They're metric prefixes, which have been standardized since the late 1700s. The prefix 'k' is used to signify "kilo", or one thousand. The prefix K is not used for anything as far as I know.
Presumably, it doesn't apply in this case because "bytes" and "bits" are not SI units. Gad, it annoys me when people such as myself get so pedantic! :-)
The one-letter abbreviations are identical to SI prefixes, except for "K" and "k", which are used interchangably. (In SI, K stands for the Kelvin unit, and k stands for 1000.) Some have suggested that "k" be used for 1000, and "K" for 1024, but this is not extended to the higher order prefixes and has never been widely recognised.
I believe the original poster with respect to k since that is what is used it the other metric units (ie: km). As for 1024, that is 'kilobinary' (or, abbreviated as 'kibibyte'): 1 kiB (kilobinary byte) = 1024 bytes.
I have frequently made the mistake of using KB when it should be kB. My personal pet peeve is when people start saying that a kilobyte is 1024 bytes and then sue companies who they think are trying to rip them off by selling them a hard-drive with 60 GB where 1 GB = 1 billion bytes. (the mistake is honest and made by nearly everyone but suing someone because of it is just wrong).
This, of course, is another example of mistakes (like the above KB) made by large software companies (such as Apple). It isn't a big deal but it is starting to become one (since iB and B are diverging noticeably, now). Some tools (such as very recent version of ifconfig under Linux, I think) are starting to report kiB instead of incorrectly reporting kB when talking about multiples of 1024.