Installing cURL from Macports fails
The issue is that you are trying to compile for the ppc architecture thus the error in cross-compiling.

Did you upgrade to Snow Leopard leaving macports on - the only way to upgrade macports across operating systems is to reinstall.

Otherwise did you choose the correct dmg to install from - it is OS dependant.

Otherwise you need to look at /opt/local/etc/macports/macports.conf and see what build_arch is set to. The default (or not set) on a new Snow Leopard macports install is 32 or 64 bit Intel code depending on the CPU you have.