I just wanted to add a few more details about my setup incase this has anything to do with it. But i have to say after having a rocky start with DNN - my sites are running better than ever with the following setup:

• Hosted on IIS7 (all system updates to Windows Server 2008 as of this posting)
• DNN 5.5.1
• RADeditor 1.1.2 with DMX Tree 1.0.1
• Using .NET 4.0 Integrated mode.
• Blog 4.0.1 RC being used currently... however 4.0.0 was providing the same results.

As I said... cannot make any new posts after the first ones made in the child blog. Nothing saves. Any thoughts?

We've been seeing similar issues with our Blog Module.  Here is our setup information:

• Windows Server 2008 R2 
• IIS 7.5
• DNN 05.04.02
• .NET 4.0 Integrated Pipeline
• Blog Module 4.0.0

Editing has never worked, I was able to reproduce this on our development server and in the database traces the calls to Blog_UpdateEntry are passing in the old version of the blog body in for @Entry.
Creating new posts with the Save and Publish button has stopped working on our production site recently.  Posts can still be made however by Saving as a draft and then publishing the draft.  I have not been able to reproduce this on our development server and I do not have the necessary permissions to trace on the production server to see what if any database calls are being made on those saves.