Question on re-leasing open source project and copyright notices

[My LinuxHAList]

Sorry, this is a bit off-topic.

Say, someone releases a project under MIT license. That project has
100 files. So, he has:

LICENSE FILE:
###
Copyright 2011 (Someone)

MIT LIcense
-----------------
[ Text of MIT License ]
## End of license file

Say, you downloaded the project and modify only 12 files and re-release it.
While, the re-release will still be MIT-licensed.

Now, in addition to Someone, you now also have copyright on the
modifications of the files.

The concise way to do it is:
# diff -r ./original ./yours > ./diff.txt
# Copyright 2011 (Someone), and portions of the modification (see
diff.txt) Copyright 2011 (you)

Is this an acceptable way ?

What is a more typical way to document such portions of copyright ?
Are "You" obligated to document such Copyright amendment ?

Thanks.