@figuerres:Yes, I can well appreciate that for larger implementations (like a 'News' website) the ability to quickly scale from minimal to as far as you need, as the demand varies during the day, would be very attractive.
I don't believe any websites' popularity can waver that much on a daily basis - while it's true a website will see increased visitation during daylight hours it's incredibly unlikely that demand on a server during peak periods would be enough to bring it to a halt and introduce a need to expand. With today's hardware you can easily support a PHP or ASP.NET website that gets millions of pageviews per day on a single $1000 Dell rackmount box - I think the limiting factor to a high-traffic website is needing a CDN for large-sized site assets or if you have a bottleneck on your database server - neither of which a VPS or VM service will help you with.
Stock-trading? That's a different matter, but those things are so mission-critical that they wouldn't be using a rental VPS or Cloud VM service.