Decryption Dictionaries
Suppose the adversary knows both the ciphertext and the plaintext for some packets encrypted with a given IV v.
This is really easy if he knows the plaintext, because, for example, he sent it to you, say via pings, or spam email!
He can also do it passively by watching for collisions.
Then he can easily determine the keystream RC4(k,v) by XORing the plaintext and the ciphertext.
Note that he does not learn the value of the shared secret k.
Now he stores that keystream in a table, indexed by v.
This table is at most 1500 * 2^24 bytes = 24 GB
Fits on a single cheap disk
The next time he sees a packet with an IV stored in the table, he can just look up the keystream, XOR it against the packet, and read the data!