I agree with Darkroom that I don't think it's necessary to invest in the second edition. You can try to work through the first edition then go to Apple's website to read up on the changes added to Objective-C 2.0 (Don't have the link to the document handy).
The primary differences between the first and second editions are:
1. The 2nd edition is Mac-oriented from the start, so it uses NSObject (rather than Object) as the root object throughout.
2. Uses NSLog almost exclusively (the 1st ed. used printf for the first part)
3. Teaches Obj-C 2.0 features like properties, synthesized accessor methods, and fast enumeration.
4. As noted, includes a Chapter on iPhone programming that shows how to build an iPhone app based on two of the classes developed in the text.
5. Is a bit more to up-to-date with respect to not using deprecated methods.
You can also ask questions about anything related to the first edition or about Objective-C in general on the forum I set up for the book at
www.classroomM.com.
Cheers,
Steve Kochan