"unusable public key" on encryption

[Douglas Choma]

I've run into a bit of a snag this evening.  I'm desperately trying to 
encrypt something using an RSA public key that someone sent me via 
email.  After importing the key successfully(???), I am still unable to 
use it for encryption.  This has never been a problem in the past, but 
it is also the first RSA key that I've tried working with.

If I run "gpg --list-keys", the key shows up as follows:
pub  1024R/3006DF31 2003-12-16 SWsoft Support Team (Generated for 
Rusoffice) <support at sw-soft.com>

So, that looks okay...I think?

Just to make sure they have supported ciphers, I check the prefs like 
this ("gpg --edit-key 3006DF31"):
(1). SWsoft Support Team (Generated for Rusoffice) <support at sw-soft.com>
      Cipher: AES, CAST5, 3DES
      Digest: SHA1, RIPEMD160
      Compression: ZLIB, ZIP, Uncompressed
      Features: MDC

But if I try to encrypt a file, all I get is "unusable public key".  
I'm feeling really lame...  :-(

Here's some system info --
gpg (GnuPG) 1.2.3
Supported algorithms:
Pubkey: RSA, RSA-E, RSA-S, ELG-E, DSA, ELG
Cipher: 3DES, CAST5, BLOWFISH, AES, AES192, AES256, TWOFISH
Hash: MD5, SHA1, RIPEMD160, TIGER192, SHA256
Compression: Uncompressed, ZIP, ZLIB

Darwin Kernel Version 6.8

Any insight would be greatly appreciated.