

You didn't mention whether or not cost is an issue.  Where
exactly are you running 100MHz??  THe digital side?  TTL?
ECL?  We run 200MHz and 100MHz all over our IC test equipment 
all day long in the ECL domain, and we use DIP's along
with PLCC's, 25mil and 20mil pitch 256pin QFP's to name
a few.  I don't see a problem in packaging as long
as you adhere to sound engineering practices.  

A good source of information is Motorola's MECL System Design
Handbook. The latest ed. is dated 2/88.  That is considered
to be one of the *bibles* in high-speed design.

The very fact that you need to build a test fixture means
you're most likely going to need a socket.  It in itself
has far more inductance per pin than the package you are
testing, not to mention any impedance discontinuities.  I
don't see the big concern over the packaging because it
probably isn't going to make that much difference

If you're trying to get TTL to run at 100MHz, have fun...
TTL was never designed to run in a 100MHz environment.
:-(

aaron