Bricking was an unlocking issue with rev 1 of the iPhone OS. It's not a concern any more.
The issue with jailbreaking is a gray one and hasn't been legally challenged in court. It's a violation of the terms and conditions of your AT&T contract. It's a violation of the warranty conditions from Apple. But both of these provisions might be invalidated by an exemption in the Digital Millenium Copyright Act allowing cell phone unlocking software.
From:
http://en.wikipedia.org/wiki/Jailbreak_(iPhone_OS)
Legal issues
The legality of jailbreaking an iPod or iPhone remains unclear, particularly in the context of the Digital Millennium Copyright Act. As part of the 2009 DMCA rulemaking, the Electronic Frontier Foundation asked the US Copyright Office to recognize an exemption to the DMCA to permit jailbreaking in order to allow iPhone owners to use their phones with applications that are not available from Apple's store.[18] In response to this, Apple filed comments opposing this exemption and indicating that they do consider jailbreaking to be a violation of copyright (and by implication prosecutable under the DMCA). A ruling on this proposed exemption has not yet been made, but a decision is expected sometime later this year.
I think Apple's arguement is a valid one - they aren't accusing jailbreakers of copyright infringement so the DMCA provision is a moot point
As JohnAn2112 mentioned, reversing a jailbreak is easy enough - one just does a factory restore of the phone (and then restores apps and media from backup).
For what it's worth, one of the very first things that I did when I got my first iPhone in late August 2007 was to jailbreak it and I've generally always had my iPhones jailbroken ever since. There's two apps that I really like that are only jailbreaking apps (backgrounder and sbsettings). But when the screen on my second iPhone failed, I fixed it myself (despite the fact that it was probably covered under warranty except for the jailbreak).