Widehead makes a good point, some apps absolutely eat memory by caching data. It's possible that the news apps might do that, especially if there's video involved, and the Market too seems to cache a fair bit. Go into settings> manage applications> all. Press menu and choose "sort by size". If, for example, channel 2 news is shown as 10mb, then there may be some data stored so click on it then press "clear cache". Don't press "delete data" because this affects the app itself. Continue on down the list app by app until you're into the 400kb apps which don't take up a lot of room. Personally I do this every day or two to maintain my memory at around 25 to 30mb.
Also check your e-mail client. I don't know about the native Mail app, but certainly K-9 mail will download an entire e-mail, including attachments. If you've received any large attachments on e-mails these take up phone memory too. I used to get some work e-mails with 5mb attachments and if 3 or 4 of these came in at once I could just watch my phone memory disappear at a frighteningly fast rate (I now use GMail to collect work e-mail until K-9 enables a "download headers only" option).
Steve.