Here's a crazy idea (and I just may go code this up for myself tonight)...
Why not let them pick whatever frequency they want??
Assuming you put your news items in a database with a datetime stamp attached, it would be a simple matter to have a server process that runs once a day and looks at each subscribed user and says "Ok, when was the last time I sent them something? Ok, now what frequency did they request? Ok, based on those two things, is it time to send them something? If yes, collect all the items from the database since the last send to them, and send it out". Good use of stylesheets and maybe some XSLT (let's say you insert your items in the database as XML) and you should get decent looking results (HTML or TXT, easy enough to make a template for either, and maybe a WAP version... oh, the mind staggers are the possibilities!)
I'm sure there's something bad about that scheme, but on the surface it seems reasonable.
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