If you plan to buy an iPhone the moment they become available, don't worry about availability. Apple says they'll have enough to meet the initial demand. After a recent meeting with several Apple execs, USB analyst Ben Reitzes reportedly had this to say:
"...[Apple is] confident the product will ship on time with volumes to meet customer demand."
Steve stated at MWSF 07 that Apple hopes to capture 1% of the mobile phone market - that's approximately 10 million units - within 12 months of the iPhone's introduction. Financial analyst Shebly Seyrafi is projecting that Apple should achieve their goal, and even sell as many as 25 million units by 2009.
Morgan Stanely expects 8 million units to leave shelves in 2007, and has raised their estimates on AAPL on that analysis.
With all of this positive press from both the financial and business world, plus the incessant chatter from future customers, it looks like the iPhone could be huge indeed.