There are several issues involved that make this not as simple as "just put the firmware image up somewhere".

First, 3rd party licensed software (Flash, etc.). The licensing agreements Nokia has with Adobe and other companies and the certification processes this software may need to go through may not allow them to distribute firmware before sales start except under certain terms.

Second, GPL software. Distributing GPL software requires them to make up-to-date sources available to people receiving the software. This is a complicated process.

Third, 1st party proprietary software. Nokia likely doesn't want pre-release versions of its own closed-source software floating about on the internet, both because it can impact perceived brand quality and because pre-release binaries may contain code which will not be in the public release.

All of these things add up to make releasing pre-release firmwares an expensive or possibly impossible proposition. Especially when you consider that all this effort will be for maybe a few dozen people at most. Sales start still seems to be on schedule for this month, waiting a few weeks may be unpleasant but it wont kill you.

Otherwise, you're free to return your device at any time for a full refund.