ISD, here's an idea that might work better than an intro page. Have it go directly to the html page, but have a noticeable link from there that says something like, "Click here to se the flash version of this site". Then, you can just link to the flash page from there. Seriously, an intro page is a bad idea. There's some research out there that says many people click away from an intro page, something you don't want your visitors doing. I think you do have another choice here. Another thing about flash vs. html before I quit: Not everybody can update flash. Some might be viewing your site at a library, school, or university on a machine that they can't control that way. You want your site to be as accessible as possible.
In general, horizontal sites don't work well, simply because people aren't used to scrolling sideways. That could potentially become distracting for somebody visiting your site the very first time. Example: The Korg
keyboard company has a site that scrolls sideways, and I never have liked it. Love the boards (I own two), detest the site. I think you've made it work in your case, though. It's just something you want to think about. Part of good site design is keeping the visitor in mind.