Grouped pipeline clobbering question

[Phillip Susi]

I'm a bit confused by the results I'm seeing with a pipeline to a 
group.  I was trying to parse a text file in such a way as the first n 
lines are passed straight through, and then a sed script is applied to 
the rest.  I thought I could do that with something like this:

cat file | ( head --lines=1 ; sed -e xxxxx )

I thought that head should consume the first line from the pipe, leaving 
the rest queued for sed to consume, but this does not seem to be the 
case.  It appears that the pipe either does not make it to sed or is 
empty when it does.  What am I doing wrong?