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...
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.
こんばんは (今晩は) is the one in the dictionary for "Good evening!". I think that こんばんわ has a cuter feeling, maybe a little softer. It's a total guess, but it might be related to the feminine わ at the end of sentences. Actually, this page seems to be saying it's related to 和 (わ - peace) which gives it a nicer feeling.