PR is only important for users and directories that rely on it as an indication of search result relevance.
For the most part, this applies to people who choose to install the Google toolbar and take the time to enable the
PR tool.
Don't pay too much attention to it since there's not much you can do about it. It rises and falls depending on a myriad of top secret algorithms and comparisons between yours and similar sites.
If you're seeing increased/sustained traffic and/or revenue, you're on the right track. Don't worry, it's all good.