It is the limit it makes perfect sense, gold is measured in copper and the copper limit is 2147483648, or as you saw in the picture 214748 gold 36 silver 48 copper. (picture had a little less)
This is 2 to the power of 31, which is an signed int32 basically + or - 2,147,483,647, but honestly because this value is used for money they really sure of used an unsigned int and effectively raised the gold cap to 4,294,967,296 copper without changing space requirements.
It is not optimized coding, but honestly most coders would of done the same thing because we are used to using int32 which is signed by default. But now that a legitimate player has actually reached the gold cap, I would imagine that they will revisit the code and change it to an unsigned int32.
Welcome to computer programming have a nice day.
This gold cap seems to be per toon not per account or per account per server. The fact that I am unlikely to get anywhere near it while Lyrana (Priest) eats cash like it is candy is just besides the point!
From the comments thread on this page: World of Warcraft - English (NA) Forums -> Did you know? Gold cap! (Pics)