People running the version of 2.1 that was posted earlier this week are running the exact same version as all of the OFFICIAL Verizon Wireless testers. Meaning, that if they for some reason decided to send out an update to intentionally brick phones running the 2.1 leak, all of their testers would also lose their phones.
Not to mention, the idea of a company intentionally designing their software to kill phones that are on their network and a source of income to them is completely contrary to their goals. Less phones on your network means less money in their pockets. Since they're a corporation and love themselves some money, they're not going to intentionally do anything that could negatively affect their bottom line.
If anything, all of the geeked out early adopters running the leak are drumming up some free PR for them and their line of newer Android 2.1 capable phones. We don't shut up about all the sweet stuff our phones can do and show anyone who holds still long enough for us to reach into our pockets. Our big mouths and nerding out saves them money in their advertising budget.
Lastly, those of us running the 2.1 leaked ROM aren't doing anything remotely illegal or wrong. None of us has defrauded Verizon Wireless in any way, or stolen their intellectual property. We're simply running a newer (not yet ready for primetime) version of a software that they're planning on giving us for free anyhow.
I know there's a lot of rumor and speculation floating around right now concerning the leak, using it and your phone exploding, bricking or simply getting up and running away in the night, but none of these are true. So far I haven't heard of a single person bricking their phone. I think Droid.Eris (the forum user, not the actual phone,) had some issues with getting the firmware to work on his phone and may have bunged something up, but if I read correctly, he took his phone to a Verizon store and they promptly replaced it.
Long story short: No, they're not going to intentionally kill your phone if you install the 2.1 leak.