particle は - Which is correct: こんばんわ or こんばんは?

I've seen both こんばんわ and こんばんは used; which is correct here? If we interpret the は as the topic particle, は would seem correct, but it seems that わ is used quite frequently anyway...

Answer

こんばんは is correct, according to that page in Japanese.

My gut feeling is the same - 今晩は ->　こんばんは.

That said, a cursory Google of こんばんは yielded 13M hits, whereas こんばんわ yielded 26M.