It would be nice if sipp could generate a raw scenario file from a packet capture. So you could create a scenario by recording a live situation and then easily convert that into a test. Obviously the resulting scenario file would need tweaks to remove retransmits and so on, but it would be an extremely helpful start.
I found that something like this already exists:
svn co http://svn.digium.com/svn/sniff2sipp/trunk sniff2sipp
I'm in the process of getting it to work, but it looks promising.