I got a Cross-Connect from Rowley and it seems to play with the Rowley environment very well. Using it, the iwIP demo seems to work well. Starts, stops, single steps, etc without oddity. I can make a change in the source, rebuild and see the changes.
I then loaded up the uIP demo, changed the IP address and rebuilt it. It seems to download OK, but if I try to run it, it gets stopped at the b_abort. I stepped to the statement where the first b_abort happened, commented that statement out, rebuilt, but it gets another b_abort shortly thereafter.
Could you load up the Rowley uIP demo onto the AT91SAM7X-EX and see if it all works fine for you? Maybe try stepping through the startup untill you get to main()?