The post not posted

A very recognisable set of concerns. For me, not posting about current research and ideas for fear of plagiarism is less of a problem - I buy into the idea that getting the ideas out there with your name attached to them is actually a useful way of demonstrating ownership. You might say that that's easy for me, as a more or less established academic, to say, and I remember spending much of my time as a grad student being paranoid about my work. But from experience of friends and colleagues who did have work pinched - in one case by an examiner - it's much harder to do anything about if it's just you and yours who know about the crime. One might hope - perhaps in a utopian fashion - that the academic blogosphere would become self-policing in such matters.
What is much harder to do, I think, is to overcome the personal fear boundary about sharing what you think are fully formed ideas with the rest of the world. It is hard to go solo if you're worried about being shot down (to use an airminded metaphor). Another issue, which actually makes this situation a bit easier for me to ignore, is that if you're writing to contract, one's publisher might get rather sniffy about everything appearing online first.
I guess the answer - if there is one - is to do what you're comfortable with at different points. And perhaps just publish a hint of what you're thinking about online and ask for comments - that way you get the benefit of other bloggers expertise without the fear factor.

Yes, it would be plagiarism. As the form all our undergraduates sign when they submit any piece of coursework reads: 'I understand that plagiarism is the use or presentation of the work of another person, including another student, without acknowledging the source. By submitting this work I acknowledge that the writing up of this assignment is my own unaided work and that where I have quoted or referred to the opinions or writings of others these have been fully and clearly acknowledged.' (Half way, nearly, through my marking now, btw). Ideas, then, not just words.
There are certainly plenty of examples of online sources being referenced in recent articles and books that I've read, and providing we stick to academic rules about referencing when we're online, I think that those who use our material will be encouraged to do the same. And it is surely easier to suggest plagiarism if an idea is datestamped and commented on (yes, I know that is probably fakable, but given the relatively small subset of academic bloggers at the moment, I think we're probably keeping tabs on each other), than if it's a thesis tucked away in a university library. In the latter case, you can tell people till you're blue in the face, but if someone else has published before you get there, you're in trouble.
Think how instinctively we search engine a new topic now. That might make your work easier to steal - but it should also mean that others interested in the same topic come across your site as well - and it's they who will call the plagiarism, even if you don't.
That being said, I recently read a fascinating profile of myself and Alex Danchev in the online journal of the Singaporean Armed Forces which seemed like the result of an in-depth interview, but which was in fact the result of extensive googling.

I knooooooow exactly what you mean about the inspiration factor. For example if you're the only person to think of researching airman ponsonby-smyth's mother-in-law's WW1 diary, and you mention it on your blog, all of a sudden everyone else knows where it is before you've published.