Today, while skimming over various WoW sites, I noticed two forum posts about the same topic: Players have discovered that there's a cap on how much money you can carry in the game. Apparently that amount is 214,748 gold, 36 silver, 48 copper. After you reach that lofty sum, you'll no longer be able to receive money from any source in the game. While some responses to the original posts claim that this exact limit had previously been theorized to exist, there have been no reports of anyone in the game actually achieving this amount via legal means.
Dorgabas on the official forums and meth on MMO-Champion's forums both reported the discovery today, each with a screenshot to provide veracity to their claims. You can check them out by clicking here. The shots are of two different players, one of whom is on a German-speaking server. In the shot you can read his conversation with a GM, which supposedly translates to him asking the GM about the limit and the GM scratching his head in response.
You can check out both threads linked above for more information, including a rough translation of the German conversation and an explanation of why this amount is the limit due to the game's coding.