Starting up in a basement and scaling

Question

So I have this 'great' idea in my head that I want to start working on and build a company around it. Been working on desktop development for 5+ years and the idea revolves around the web. My exposure to web development is purely academic(a few database driven websites etc).

I know I have the wherewithal to get started and build something that works but am also sure that I don't have the expertise to scale whatever I build to huge numbers.

Looking at how some of the web majors started in dorm rooms etc, I am wondering how most web startups approach this problem (I'd find it difficult to believe that all the major startup founders had something that would scale to millions of users in their first jab).

Do you think its wise to run with whatever I can and then worry about scaling if and when its necessary? Or do you think its a sign that I am trying to bite more than I can chew?