I claimed rights to a website and a magazine idea I had in some threads I started on this site. I live in Virginia. Does just saying I have rights to these ideas- and threatening to sue if they are stolen and I find out :D- give me legal rights?
No one has any rights to ideas.
You can copyright an expression of an idea. You can patent a device that uses your ideas. You can trademark a slogan.
But the idea itself belongs to no one. You have to take the next step to get any sort of rights.
…and if you mentioned it here, you don’t even have copyright on the original posting. One of the SDMB terms of service is that everything we write (such as this timeless prose) belongs to the Chicago Reader.
Actually, Mr. TimeWinder, that’s false–you idiot! Copyright, of course, remains with the original author. But the terms of service DO say that we’re granting the Chicago Reader the right to do what they want with it, which isn’t quite the same as giving them the copyright.
I bolded the two sections you specifically mentioned. It’s at the bottom of every SDMB thread, for future reference. 
RealityChuck is right: Nobody can own an idea per se, only a specific expression of that idea in a specific medium (copyright), as a specific invention (patent), or as a specific identifying mark in a specific commercial field (trademark). Copyrights and patents both wear off after a specified period of time (in theory) and trademarks wear off after the holder has failed to enforce them ‘well enough’ for ‘long enough’ (in the view of a court).
But no one can assign themselves rights by so simple a method. Permission for copying a copywrited work can only be given in writing (the famous “express written permission” of televised sports). Chicago Reader Inc. may have a claim to “fair-use” if they decide to publish these threads, but if you write a brilliant poem that they published separately, they would be violation your copyright.
-Saint Cad
By responding to this post, you give me all rights to rifle your bank accounts, seize your personal property, and sell your first-born to passing Gypsies. I get exclusive rights to pick the food you eat, the clothes you wear, and the color you paint your house. You may not use any any portion of this post for your own use including the individual letters ever again. This means that you must spend the rest of your life using the crappy letters like “J” and “Z” and “7”.
Ah, but they CAN publish your poem based on their disclaimer. They can’t make a unilateral declaration of rights, but they CAN provide you with terms of use for their board. By joining and posting, you have agreed to their terms of use and formed a contract with them. It is thus no longer a unilateral declaration of rights but a contract.
This, on the other hand…
does not constitute a contract even though I read it and responded to it because you have provided me no goods or services, which is a crucial part of a contract. The Chicago Reader allowed me to read your words. You did nothing for me in exchange for which I would be expected to allow you to seize my property. Although if you really want it you can have my couch. It’s getting stinky.
A little more research.
An exclusive copyright can only transferred in writing under US law. Even if self-granting of rights were legal (I would still question this), it cannot work with exclusive copyrights.
Non-exclusive transfer of a copyright does not need to be in writing and clearly this is what SD is counting on. I emailed the Copyright Office to clarify whether a disclaimer can be used for such purposes. More to follow.
I provided you with a demonstration of my supernatural brilliance. When are you going to send me your first born? 
Way back when I took a semester of law some part of the consideration had to be money. In Iowa at that time contracts started out, “For $1 and other valuable consideration yadda, yadda, yadda.”
Well if you really want her, send money for a bus ticket and she’s yours! However, I must point out once again, that YOU did not regale me with your brilliance: The Chicago Reader kindly shared your brilliance with me. They indicated that they had no room for an extra bed in their offices and would rather my daughter stayed home.
As to the disclaimer at the bottom: At no point does the Chicago Reader claim copyright for your post. They are claiming a compilation copyright (i.e., all the posts as a unit), but nothing about the individual posts.
In addition, you grant them the right to reuse your posts without compensation. If they were claiming copyright on the individual posts, there would be no need to do this.
As a matter of fact, their disclaimer is a pretty good one: if someone takes your post and puts it on their web page, it’s their lawyers who would get involved.