Piper Jaffray’s Gene Munster freely admits that doesn’t really know what Steve Jobs has up his sleeve for the special event Apple (AAPL) has scheduled for Sept. 1. But that doesn’t stop Munster from speculating — both about what products and services might be unveiled, and what their unveiling might do to the company’s share price.

In fact, in a note issued Wednesday, he advises clients to seize this opportunity to stock up on Apple shares:

“While not an indication of future performance,” he writes, “shares of AAPL have increased, on average, 12% in Sept. over the last five years (excluding 2008 when the market saw widespread declines)… Similarly, AAPL stock price increases 22% from the beginning of Sept. through the end of Dec.”

Caution: to make those averages work, Munster has left 2008 out of the calculations entirely. See chart below:

As for the new products and services that might trigger a rise in share price, Munster’s list is not much different than any rumor site’s:

• New iPod touch. “With a back-facing camera for still images and videos and a front-facing camera for FaceTime video calls.”
• Lower-cost iOS-based Apple TV. “Bringing the App Store to the Apple TV would add many new and meaningful features (like Netflix streaming, or video games, for example).”
• 99-cent TV show rentals. “We would view [that] as a step forward for Apple TV. We do not have any knowledge of a new Apple TV, but we believe it would fall into company strategy and product cycles nicely.”
• Cloud-based iTunes service. “We believe an announcement at the Sept. event is likely, but the service may not begin until late CY10.”

[Follow Philip Elmer-DeWitt on Twitter @philiped]