googling こんにちわ brought me to this page
http://park15.wakwak.com/~o0o0o0o0/bokumetsu/ (annihilation of "konnichiwa" ??)
Quite interesting. From the site besides cutesy, it appears that expressing youthfulness is also the reason why people use it, since many young people use it. But, while maybe it's ok to use it in casual mail with with your friends, it could be very rude if you use it, say, in email to your teacher or in another formal situation, since it's not officially accepted. I think same goes to こんばんわ as well.