If you have not set a Default Domain for the IP address, then a generic Plesk page should show in the browser.
Can you ftp to the domain name instead of the IP? <-- Nevermind, I just re-read your post.
Have you setup anything in the firewall which may limit access to some IPs?
Are you using the Plesk Firewall, generic Iptables, something else, or nothing?
I just saw your post on WHT and your Plesk DNS settings look fine. Have you run a report at:
http://www.dnsreport.com
Also, when did you submit the latest DNS changes for the domain? If it was just recently, then DNS may not have had time to propagate fully around the world to all other DNS servers yet.
If this is the case, then the DNSreport.com should reflect errors or incorrect IPs.
Please PM or email me with your domain name and what the proper IPs are supposed to be.