dumb question: tracking kernel back to sources that built it.

[marty fouts]

I'm set in my ways. One of the things I like to be able to do before I
start whacking away at a kernel is be able to build the installed
kernel from sources. But I seem to be having trouble doing that with
Ubuntu.

I've installed the 2.6.15-23-686 kernel on a particular box. Then I've
grabbed the linux-source-2.6.15 kernel source tree.  The config file
that's installed with the 23-686 binary tries to compile some obscure
usb driver that won't compile, which makes me suspect it is not the
right config file. The config file one gets by default from the kernel
tree isn't for an smp, so I'm sure it's not the right config file.

I've tried the irc channel and the forums and even a lot of googling,
but I've clearly missed the steps I need to take in order to be able
to reproduce the kernel from sources.  What am I missing?

Thanks