I am not sure if there is an extension similar to this but could not find it. Either way this was created by myself and someone requested this earlier today.
This extension is fully compliant with W3C and was built with extension builder 2.0.2
PDF Included in ZIP file.
UPDATES:
09/11/2010 - ** Latest Update **
Rebuilt with EB 2.5.2 to to work with WB7
16-08-2010
Fixed a bug that could cause redirect problems for some users.
To update simply overwrite the two old .wbx files with the two
new ones and reupload the webpages that uses the extension.
12-01-2010 -
Official Extension Release
Download:
Click to go to download page on my site.
Demo
Due to large amounts of spam and abuse of the demos, all demos have now been removed. I do apologize for the inconvenience ... but i cannot afford to loose valuable bandwidth and cause server overload due to heavy spam on the demos. Genieuk does not earn any revenue from the Genieuk website and as such cannot afford to loose valuable resources. To test any of my extensions please just download and install in Web Builder and test the extension(s) this way.
What this extension does?
This extension comes in two parts. If you have a website or webpage where you want a visitor to agree to some sort or terms and conditions or terms of service etc before accessing any particular webpage then this extension is what your looking for. You could protect your entire site from being viewed or just certain page/pages. It uses a Cookie so if visitor revisits at a later date they wont have to re-confirm. The Cookie is set to last for 30 days. It is made so you can place the i agree (form) button on any page you wish so everything stays blended in with your site. It also comes with customization so you can make the button blend in with your other site buttons for example.
Part One of this Extension
You simply place the I Agree Protect Page extension on every webpage you want a visitor to agree to for example terms and conditions before viewing.
Configurable Option(s)
This part of the extension comes with 1 configuration option and that is what page to redirect to so they can confirm they agree to for example terms and conditions.
If they not agreed and try to access the webpage directly they will be instantly redirected to the webpage where they must agree. The extension uses a cookie and it lasts for 30 days so if they revisit at a later date they wont have to re-confirm and can go straight to protected page.
Part Two of this Extension
The second part of the extension is the i agree button.
You simply place the I Agree Form extension on the page that user has to confirm to agree to for example terms and conditions. It has been made so you can place it on any of your webpages so for example you can place on one of your webpages that uses your website theme. If a visitor hits the button to confirm without checking the box they are given an error message.
Configurable Option(s)
1) Change button text. Default is I Agree
2) Change font of button text
3) Change font size of button text
4) Change button text color
5) Change button background color
6) Change button border color
7) Change button border width
8.) Change button border height
9) Change error message text if users clicks button without checking the check box.
10) URL to redirect visitor to if visitor accepts terms of service for example.
Here are some images as an example.
This page is where the form has been added. As you can see it can be anywebpage so it fits in with your site theme plus you can change button style to suit your site. I just used a template in web builder and added a text box with some terms in for an example. You can obviously put anything you want.

If visitor clicks the button without checking the box they are given an error message. The error text can be changed by you. The error message will appear under the button. Just make sure the form button box is wide enough and has enough height so the error shows.

If the visitor checks the box and clicks on the button they are taken to a protected page (specified by you) that can only be accessed if i have agreed to terms. If i tried accessing the protected page without agreeing i would be redirected to the agreement page unless of course i previously agreed which a cookie would have been set so i am not asked to confirm again.

The page below is just an example of a non protected webpage. I can access it without agreeing to any terms etc.

Obviously you could use this for dozens of purposes. Like a-dult content webpages or for example your entire website.
Some countries for example require website owners that have content for a-dult viewing only etc to display some sort of terms etc, well with this extension you could do that making sure you are abiding by any legal terms your country may have that you should respect, or perhaps you have a downloads page that you want user to agree to terms of download etc... I think you get the idea of all the possibilites this extension can be used for.
Hope you enjoy and please leave feedback if you like

Regards,
Mathew