I never heard of forming a separate LLC for each flip. Setting up, doing the paperwork, opening a separate checking account, paying the accounting fees and abiding by all the LLC rules and regulations seems like it would be time and cost prohibitive.
I have heard of investors setting up a separate LLC for each rental property in their portfolio, but even that is deemed overkill by some BP members.
@Ben C. If they are single member LLCs, they all end up on your personal tax return anyway (schedule E). This shows your income over time.