This only needs to work on a single subnet and is not for malicious use.  
I have a load testing tool written in Python that basically blasts HTTP requests at a URL.  I need to run performance tests against an IP-based load balancer, so the requests must come from a range of IP's.  Most commercial performance tools provide this functionality, but I want to build it into my own.
The tool uses Python's urllib2 for transport.  Is it possible to send HTTP requests with spoofed IP addresses for the packets making up the request?
Quick note, as I just learned this yesterday:
I think you've implied you know this already, but any responses to an HTTP request go to the IP address that shows up in the header. So if you are wanting to see those responses, you need to have control of the router and have it set up so that the spoofed IPs are all routed back to the IP you are using to view the responses.
Spoofing the origination IP address of an HTTP request