The loaner policy varies from store to store. The Apple Store I work at in NJ has the following loaner policy:
1. The loaner is free if the problem is due to an apple manufacturing defect (e.g. your iPhone stops charging, it won’t sync with your computer, the backlight stops working,etc)
2. You have to pay a $ 15–30 loaner fee if the problem is due to some kind of mishandling on your part (e.g. you dropped the iPhone, you have sand in the iPhone, your friend sat on it, etc.)
You can get a loaner instantly if you take it to a Genius bar near you. But if the case is that Apple has to ship it to you, then you have to pay the shipping fee (which will cost around $15) even if your iPhone qualifies for a free loaner .
With the iPod touch and other iPods, there are no loaners. If your iPod breaks down, it will be replaced with a new (and sometimes refurbs but you wont know the difference) iPod for the most part (unless you require a new battery)
Also a word of advice, waiting for Apple tech support via email can be lenghty process. You’ll get support a lot faster if you call the Customer Hotline or just bring it an Apple Store.