Firstly the download will be approximately 4GB in size. Even if you have one Mac that could be a problem. Some people have data caps on their broadband e.g. Sky and believe me 4GB is large, heck some people don't even have broadband! Apple will probably say go to an Apple Store and use their free Wi-Fi - not going to happen as everyone can't get there and it's still going to take an age to download. So you have broadband and you (think) you have unlimited data (hey didn't you know "unlimited" is not actually unlimited?) but you have four Macs in the family home. So you buy Lion once, download it, install it on the first Mac. Then go to the next Mac and download it using the same account, that's 8GB of data, third Mac at 12GB and so on up to your 16GB of data.
Now there are various reports (including instructions) how to copy the current beta version of the Lion installer to a disc and use it to install on every computer. Maybe Apple will let you do this maybe they won't (either legally or physically - the reports are only for the beta version of the Lion). Even if Apple do let you do it it may not be a trivial process.
So will Apple sell a physical disc of Lion upon it's release or later? This is Apple so they aren't saying right now!"
So what's the problem? The real issue here is for businesses, commercial, education and/or site licence users. Straight off the bat you can only (legally) do the multiple install if you are a "consumer". If you are business/commercial user you can't do that. Even if you can copy the Lion installer on to a disc you're not legal (and the BSA will be all over you ass in this country - thanks Apple!). Wanna be legal? Well you will have to create an individual iTunes account on each and every computer to then separately download (at 4GB a pop) and install Lion. This just isn't going to work!
Now I'm sure Apple will have a solution for businesses, commercial, education and/or site licence users (eventually) but it sure would be nice to know what it will be…
Update 20/06/2011 - Apple has released a US only (so far) PDF with information on how businesses, commercial, education and/or site licence users can buy Mac OS X 10.7 Lion. See our Lion client / Lion Server FAQs for more details.