serial number for a product that has been replaced

Well... if it's working for you, you don't need to do anything.

You have no warranty, though. It can not be serviced by Apple.

If there is some kind of problem with it, you will need to seek out a 3rd party repair shop.

That's correct. There is no warranty on that phone. You purchased a gray market phone that was lost, stolen, replaced for some reason, and then re-sold through illegitimate channels.

IF you believe you purchased it from a legitimate, authorized source, then you need to go back to them and find out what's going on.

but it is no longer under warranty... not sure if they will entertain my request... I just wanted to add in to my list of registered product cos I bought another new device recently... and discovered this

All you're asking them to do is correct the records in the database. Either someone made an error or they sold you a stolen phone. If the problem is not corrected, you will never be able to get that phone serviced by Apple if it's necessary.

That indicates that the phone was replaced and then re-sold illegally. It may have been reported lost or stolen.

Where exactly did you purchase this phone?

Is the local telco an authorized carrier of the iPhone?

Are they on this list? http://support.apple.com/kb/ht1937

I encountered the same problem but my phone was bought from local Telco

Yes it is listed...

Then take it back to them.

I dropped the telco a message about this via their online feedback form. Will see how it goes. But I sure hope I never would need to service my phone

The telco referred me back to Apple after doing some preliminary checks.