We could probably discuss this spelling for ever. Somebody has even created a web page about it. 
According to The American Heritage® Dictionary of the English Language, Fourth Edition, the most widely accepted spelling is a cappella. Many web pages    and the A-Cappella Music FAQ   agree.
Some writers    use a cappella in their own writing, reserving a capella for quotations, titles and meta tags, acknowleding it to be incorrect. Others  , and The Columbia Guide to Standard American English, acknowledge what people are actually writing and hedge their bets, accepting a capella as a valid alternative spelling. The Merriam-Webster Dictionary takes this position, yet also confirms that Capelle means goat and is the name of a star.
Some apparently authoritative sources also concede  acappella as a most popular de facto usage.
- In truth, a capella means in your hair, a cappello means in your hat, and A Capella is an astronomical reference to the first planet circling the star Capella in the constellation Auriga.
The popular vote: Google finds 247,000 hits for a capella, 205,000 for a cappella and 77,600 for acappella.
EdH 15:34, 3 Aug 2003 (UTC)