There have been several successful attempts at hacking dnn sites, some of which have been due to vulnerabilitie, some which are control panels like plesk and a couple of others I can't remember off the top of my head.
I wrote a blog about this last year - http://blogs.ninameiers.com/2006/05/05/You+Better+Take+That+DNN+Security+Breach+SERIOUSLY.aspx and I posted earlier this year about the title in the name - which has now been removed by default -
http://blogs.ninameiers.com/2007/01/23/Are+You+A+DotNetNuke+Sitting+Duck.aspx
And in most instances they are mainly nuisance hackers - they delete you default.aspx file and things like that - or they put in an index html file.
I believe the forums here have the signatures locked down for now since there is a security breach, but I guess it means anyone else who's using the core forums may suffer from having their sites compromised. That occured when the mothership herself was attacked and showed a very similar page to your for a short time. I think they mighth be related but can't be 100% sure.
Read that blog, also, check that you're not missing the default.aspx file, which will bring up the custom error page. And contact your hosting company and see if there are any other reports of their server being compromised.
You can upgrade if you want, but at the very least you should be upgrading to one without a security problem. Fortunately DNN is more secure that some other apps I've seen, not perfect, but nothing on the internet is, and although these issues caused grief to many people, they were easily fixed and more nuisance in value.
See if you pick anything up from there to sort you out and I'm sorry it happened.. it's very frustrating and a waste of time for all.
Nina Meiers
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