I see phrases like 200人ぐらい, 半々ぐらい, and 何分くらい, which seem to indicate that くらい and ぐらい are synonymous, if not interchangeable. Is there any kind of rule for deciding which to use, or is it a stylistic preference?
This page at the goo.ne.jp Q&A site quotes the NHKことばのハンドブック, which states that while there were at one time rules for when to use くらい and when to use ぐらい, modern-day Japanese has no such distinction. I agree with Tsuyoshi that ぐらい "feels" colloquial, but more often than not I think it's a matter of which rolls off the tongue with the most ease.
I've been told by at least a couple teachers that the word is typically pronounced ぐらい, but is properly written as くらい, similar to じゃありません versus ではありません. So for proper-ish documents, you'd want to use the latter, but informally the former is fine.