As far as I understand it, the 1.8T model was initially labelled a GTI with all the letters being silver which you still see plenty of on the road. The 2.0 litre model was just labelled '2.0'. All this was as it was was done on the continent, but then apparently a lot of 2.0 litre owners over here were requesting that they wanted a GTI badge on theirs (there's a good reason why it didn't have a GTI badge!). So eventually a lot of dealerships started re-badging them. so THEN the dealers had to do something to distinguish the 1.8T from the 2.0, hence the introduction of the red 'I' which appears on the more recent 1.8T's.
One important thing to note is that all the re-badging is done in the UK, they are all shipped from Europe with the original badge schemes and it's the dealers over here have to change it all to please the petrol-heads before they are delivered. The confusion arises from the fact that some dealers obviously stick with the European convention and don't re-badge anything, hence people still get all kinds of variations. Like the dealers that Frosticles spoke to seem to still go by the european convention.
According the 'UK scheme' your PD150 should be labelled 'GTI' with a red I (and so it should as it whips the 1.8T on the road). When I got my PD150 delivered I was fully expecting a TDI badge but was very pleased to see 'GTI' sitting proudly on the back! Incidentally, I once saw one fool in Spain had his labelled as a 'GT TDI 4Motion'!!
One visual clue to distinguish the 1.8T and the PD150 GTI's is the PD150 has two 'air scoops' on the plastic below the front bumper.
