Something you need to consider is the "use" of bitcoin. As a retail purchasing product, I don't see it getting that high.
BUT
If security eventually gets locked down, and people scared of their country's own currency (greece, spain, new jersey), and they see bitcoin as an alternative holding product (like gold), then it could go really high. One scare in their country, and they don't care what the price of bitcoin is, they are going to convert as much money as they can into bitcoin.
Now that is a different story for bitcoin continuing to grow and grow.
: )
cj
The price of a whole bitcoin is irrelevant "as a retail purchasing product"....I would say that for most transactions it already is irrelevant...unless you are spending thousands on a product you are already dealing in Satoshi's.
By the time if/when it ever gets to the $1m level it will be regularly denoted in Satoshi's. A cup of coffee from Starbucks is 35 Satoshi's etc.
You will probably use them without knowing or realizing that you are buying and selling them on the fly as you need to.
You will just be using your debit/credit card as usual....in the background all the processing and conversions (if needed at that point) will be performed.
All online transactions will be done in XBT in the interest of security...and government taxation/tracking...once that happens they WILL be worth an enormous amount of money. How much is anyone's guess.