I had the same problem, using an IBM thinkpad T30. It seems that for whatever reason pump, and ifconfig both lose the ability to set some flags using ioctl(), or something like that.

My workaround was to kill pump, remove, and reinstall the kernel modules for my NIC, configure my interface with iwconfig, then restart pump.

For example,

pump --kill
rmmod orinoco_pci orinoco hermes
insmod hermes;insmod orinoco;insmod orinoco_pci
iwconfig eth0 essid "network" key restricted 0123456789ABCDEF0123456789A
pump

I hope that helps.