I've been working on a WebApp that will eventually be a one stop shop for your notification needs! Its pretty limited right now, but it will push twitter feeds, twitter trends and some shipping notifications (UPS and USPS) to the iPhone via Prowl. I'll build it out over time, but maybe some of you would enjoy it as-is.
Update (1/6/10) - I've made some stability improvements as well as added a few features, including much better support for Twitter. It's now using the Twitter Search API, so (most) anything you can search for (hash tags, location, to and from, mentions, etc) you can set up a prowl notification for.
Update (1/14/10) - I got tired of not having fine control over the notifications on the iPhone, so I added in priority controls. I've also tweaked the notifications a bit so each type (UPS and USPS Shipping, Twitter, and Twitter Trends) have unique Application names, so users can control redirects easily.
Update (1/20/10) - I've added in generic rss feeds to the notification types. Since Prowl doesn't render html code, the notification only includes the title of each feed item, but the notification includes a shortened (by bit.ly) link to the original article.
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