So, scratching an itch some time ago (July 12., according to my timestamp), I went ahead and wrote a notifier for Reddit based on the simple but awesome Facebook notifier. Then it hit me that maybe there were other Reddit and KDE users that would appreciate this, so I uploaded it to my server.
Download
You can download it from here: http://mts.ms/redditnotifier.plasmoid
To use it
- First add it to the panel or wherever you want it.
- Right click on it and choose “Reddit Notifier Settings”.
- Go here and get the URL for the feed you’d like to monitor (I use the “everything” feed), and paste it into the appropriate field.
UPDATE
I put up the source on github, and uploaded it to kde-apps.org so that it can be easily installed directly from Plasma/GHNS.
sandsmark's blog 
Hey, I wrote one too. Mine’s only at the level of “works enough” though because I don’t actually use it. But feel free to take some inspiration.
https://github.com/Falmarri/reddit-plasmoid
Looks quite bit more complex, though, mine is just a couple of qml files.
I don’t intend to troll, but, wouldn’t be better to concentrate on a generic RSS plugin for Akonadi + RSS dataengine + generic RSS plasmoid? That way I would have live notifications for Reddit, and anything supporting RSS.
Thought about it, but I just wanted something quick and dirty that I could use right away, and I honestly don’t have many RSS feeds I follow. And I have enough long-term big-investment projects atm., if I’m not going to start working on Akonadi as well.
@Ernesto: There’s already an official KDE plasmoid RSSNow, along with a data engine like you said (this plasmoid, in fact, uses that data engine). The problem I find with it, personally, is that only one post from each feed is shown at any point, you have to click the arrow buttons to browse through other posts. This plasmoid shows all posts as a vertical list in one plasmoid space, I can see the 7-8 latest posts from one RSS feed and scroll down to see previous posts.
@martinsandsmark: Good job! (Y)