I bought a Bullet last month in Nepal. Carnet issued by the RAC in London. No problems with India, Pakistan or Iran borders and no bribes needed either. Harvey is right in that the bikes are more expensive. Without the exchange rate I forget - but think about 50% more.
I do know there is another Dutch guy currently in Iran (Bart - he's on HUBB) who, with two others, has made it through on Indian bikes with no Carnet. However, for three bikes, I think it cost $150 India, $150 Pakistan, $100 Baluchistan and $180 Iran in bribes to get through - plus several hours at each border.