I'm going to give you an idea of how Apple releases products. This might help with your decisions about when to buy.
There is no exact science to this and careful attending must be take to find out whether to purchase an Apple product of not. Here I will give you a brief guideline. If you ever want to know if you show buy or not just email me and I will give you some specific help.
Apple generally has about 4 conferences a year. This can change at any time but there is one conference that happens annually and is always a guarantee. That conference is World Wide Developers Conference or WWDC, this normally happens at the beginning of July but in 2010 it took place in June. There is speculation as to why the change but will have to wait till next year to see how true it is.
Always remember that Apple generally only does major updates to products once a year, so if the update has already been done don't expect another one.
First: The first conference normally happens in March or April. There could possible be two conferences durning this time. The most important will be major updates to the Macbook and Macbook Pro or Macbook Air line up. Minor update can be done without a conference. The second conference with less importance is a preview of the new iOS software that will be released during WWDC.
Secondly: WWDC is a guarantee and will bring a new iOS and generally updates to the iPhone and the iPad in future. As the iPad is a new product will might take some time for it to fall into this product life cycle.
Thirdly: During September Apple will almost without fail release new iPods before the holidays. This will include updates to the iPod Touch to fit with the new iOS software that was released at WWDC.
Fourthly: During October Apple has been known to do updates to the Mac, this includes the iMac, Mac Mini, Mac Pro. Generally the iMac gets the most attention as it is the most popular desktop. The Mac Pro is already the best desktop in the world so leaves so has little more needed as it's doesn't come with a screen. The Mac Mini has see very little attention but I suspect a major update this year.
As I have said before this is not conclusive but a guideline. Let me know if you want more specific advice.