I've been asked this a couple of times by a number of people since RC1 came out. I experienced this myself when I was trying to do a AirCanada Web Checkin to print my boarding pass last Friday before heading out the door of the Microsoft office to catch my flight home. It also affects some Canadian Online banking sites as well.
**Paragraph below edited for clarity - edits in italic**
It is all about security. SSL security that is. IE 7 in Vista RC1 has SSL v2.0 turned off by default in favour of SSL v3.0. When IE engages in an SSL handshake, it is going to use newer / more secure SSLv3 which includes TLS extensions. Some older servers do not support TLS according to the RFC and they "break" when when they see the TLS extensions. That's why you see the "IE Cannot display the webpage" error. By re-enabling the SSL 2.0 checkbox (or turning off the "Use TLS 1.0" checkbox), IE 7 will no longer send the TLS extensions during an SSL handshake and the webpage appears as normal on these older servers. It's interesting to see who is still running with the older (but still secure) SSLv2 out there. Don't worry - if you don't make the change below - you will find them out yourself. Microsoft is working directly with site owners to help them update their servers to support TLS extensions.
The change is quite simple. This is a temporary work around you can implement during the Beta. Launch IE and go to Tools / Internet Options / Advanced Tab. Scroll to the bottom.
Here's a screenshot of the default settings.
Turn on the checkbox for SSL 2.0 and all is fine. I hope this helps.
P.S. I took it on my new Toshiba M7 Tablet running Vista RC1 using the built in Snipping tool.
**Editor's Note** - I edited the paragraph to give some more details and provide some clarity as to what exactly was going on. The blogsphere is great for getting information out to people quickly and that includes getting feedback to the post authors. Thanks Scott and Eric from the IE team!