Spoofed FTP connections

[johnscimone () HOTMAIL COM (John Scimone)]

When setting up a windows based ftp server yesterday I 
noticed the option to enable ip checking for certain 
accounts.  What I was wondering is whether any ftp servers 
that use this feature are vulnerable to spoofed packets of 
a legitimate user telling the server to accept a connect 
from another computer(the client sending the spoofed 
packets), because isn't the forwarding allowed on most 
sites because thats what FXP does to enable FTP to FTP 
transfers?  Combining the spoofed packets and the forwarded 
connection type option couldn't a client avoid ip based 
authentication?  Also if this is possible has anyone 
written an FTP client yet that performs this?  This is 
totally theoretical and I'm sure I'm probably missing 
something which makes this impractical, but any replys are 
welcome.