I run into trouble using GDB 7 (7.3) on MacOS Lion.
What I did was:
$ sudo port install gdb
After that GDB was installed to: /opt/loca
Try cgdb for now, it's actually pretty nice so far:
sudo port install cgdb
I don't claim to understand why the latest gdb sources have an issue on the mac, but I have the same issue as you and I tried cgdb and it works for me. Links for more information: