As Long As vs. So Long As: As You Like It

I received an e-mail from Ryan with a link to a story he had spotted on MTV.com that featured the following headline:

Lollapalooza: Where Moments Are Made, So Long As It Doesn't Rain

Ryan asked,

"I always thought the expression was supposed to be 'as long as,' not of 'so long as.' What is the rule?"

Both expressions are subordinating conjunctions. There are some cases where you'd definitely use "as long as" and not "so long as" (see below). But this isn't one of those cases. The way "so long as" is used in the MTV headline is fine; "as long as" would also be fine. It's simply a matter of preference.

Building blocks:

1. A conjunction joins two parts of a sentence.
2. A subordinating conjunction is used when there are two unequal clauses. The main clause is dependent on the other clause. If A doesn't happen, neither does B. I'll go, as long as it doesn't rain. 
3. The subordinating conjunction begins the dependent clause. 

Quick rules: 

• Use either "as long as" or "so long as" when you mean "if," "provided that" or "on the condition that." I'll go, provided that it doesn't rain. I'll go, so long as it doesn't rain. I'll go, as long as it doesn't rain. All three examples work equally well.
• Use "as long as" when comparing length. The pencil was as long as the paper.
• Use "as long as" to indicate a duration of time. I'll stay as long as you need me. 
• Use "as long as" as a replacement for "since." Since you offered, I'll take another slice of pizza. As long as you offered, I'll take another slice of pizza.