There's no such thing as 'auto-magically' unfortunately, but responsive webdesign can help make a desktop targeted website behave more mobile friendly. Nothing beats a truly mobile created/targeted website but often budget (and/or time) doesn't allow this.
Initially responsive webdesign will cost you extra time, but after 2-3 projects things start to become 2nd nature and when you reach that stage, you'll be ready to offer a better user experience across devices for the same amount of time spent developing.
A 4 column design for a blog or showcase may look good on a desktop, but on a mobile device it might be better to 'transform' it into 4 rows of 1 columns or 2 rows of 2 columns... or perhaps don't show (for instance) that youtube video at all. That's where Sitefinity's mobile features truly come in handy.
And if Sitefinity's column reflow and its rule-sets are to limited for your design, you can still add your own styling/design rules and preview them on the range of mobile devices.
To use a car analogy; 10years ago we had sport cars and trucks - these days we have SUV. They're not as fast as a sports car and can't transport as much as a truck but they're the overlap of the two and offer a cross experience for the price of one.
I just ran a test on http://www.w3org/Mobile/ for a new responsive site I'm working on and I'm scoring 91% and hitting 4 low level errors.
Two are related to it not understanding html5 doctype/utf-8 declaration. One is the page exceeding a '13Kb' limit and the last is not proper declaration of width/height attribute with images. And there's a warning regarding the use of scripting.
So even a 'perfectionist' like me ends up with errors, don't be thrown off by them - let them help you rethink your solutions IF
you want to.
Sports cars and trucks are still the best for both worlds, but if you don't have the time or budget and want to offer a more cross device (phone/tablet/desktop/tv) experience Sitefinity Mobile's features are the smoothest way to start.
3 great starting points:
Responsive Web Design
by Ethan Marcotte
by Andy Clarke
Building Hybrid Mobile Applications