port 6051: hacked?

[Ramin Motakef <ramin@motakef.de>]

Hi all,
Todays nmap run shows me:

Interesting ports on  (xxxxxx):
(The 59984 ports scanned but not shown below are in state: closed)
Port       State       Service
21/tcp     open        ftp                     
22/tcp     open        ssh                     
25/tcp     open        smtp                    
53/tcp     open        domain                  
80/tcp     open        http                    
110/tcp    open        pop-3                   
111/tcp    open        sunrpc                  
143/tcp    open        imap2                   
199/tcp    open        smux                    
389/tcp    open        ldap                    
443/tcp    open        https                   
993/tcp    open        imaps                   
995/tcp    open        pop3s                   
3306/tcp   open        mysql                   
5432/tcp   open        postgres                
6051/tcp   filtered    unknown    <--------------                 

fuser -v 6051/tcp gives no result. 

Questions:
Am i hacked? 
Is there any other way i can tell which program is responsible for
this port?
What exactly is the meaning of "filtered"?

 
Info: 
System is running unstable, last upgrade 07-02-2002. Kernel 2.4.5. 
chkrootkit (latest debian) shows nothing strange .

Thanks,
Ramin