Is there a way to restore the iPhone's oleophobic screen coating once it's worn off?

No. The organic material is applied during assembly. I'm not aware of any place that can "replenish" the lipophobic substance.

Apple has this to say:

"Please note that the ability of this coating to repel oil will diminish over time with normal usage, and that rubbing the screen with an abrasive material will further diminish its effect and may scratch your screen."

Source: http://support.apple.com/kb/ht3226

You can, however look into replacement covers that also contain lipophobic properties: http://www.mobimega.com/sgp-steinheil-ultra-oleophobic-screen-protector-for-iphone-4.html

Or pay for a new glass screen. Warranty will generally not cover this, but you can always ask for a quote from Apple service in or out of coverage.

There is a product called Fusso oleophobic coating (available here), that promises to restore the oleophobic properties of your iPhone display (there is also a tablet version)

I personally haven't tried it, but the website ifixit.com, specialists on repairing Apple products, say that the product works really well.