Closed source until beta/release to protect the coin from clones while it is being developed. Clones are not bad, and it can provide a legitimate competitive rivalry among developers, but I think doing it this early would be a mistake pragmatically due to greed and capitalistic manipulation.
After that, it must, must, MUST be open source -- the community is for the most part somewhat more knowledgeable in cryptography and the need for anonymity in transactions, and you simply cannot have this if it is closed source. Anecdote: this is why Truecrypt is infinitely more popular than any other data encryption. No matter how good your intentions, no-one is going to trust a closed source client these days. No one even trusts "trustworthy" companies like Google with their data as they did all doe-eyed in 2008 with their (in)famous "do no evil" motto when they, too, simply bend to the will of their masters.
I think it should remain closed source a while after going live with it. Some people are going to use the plugin no doubt, even though most wouldn't. However, the promise that the source will be released soon, and the fact that it is working, will affect the market price positively (and thus general interest towards the coin). And also it creates yet another climax point when there will be countdown to the release of the source etc. All about marketing.