No, the tip of the original iLiad-Pen does not seem replacable.
However, it's a wacom-Pen with a touch-sensitive tip that tells the device when the tip touches the surface and when it's just hovering. (iLiad owners can try this, just hover over the display and press the tip with your finger)
There are other Wacom Pens that work like this, some of them have a felt-tip and some are said to work with the iLiad.
I don't know whether there are pens that do both :-)