According to the unofficial terrahash pre-order google docs spreadsheet we can see that people have posted 175 orders to the spreadsheet.
122 have paid
5 are in unpaid status
27 have canceled
For this completely unscientific study I am going to lump the unpaid in with the 27 that have publicly canceled.
So we have 32 that have canceled out of 175 which gives us 18.2% cancellation rate, I would say that this is quite high for a start-up.
In addition we also have numerous people on the bitcointalk.org forums that are selling their pre-orders probably in an attempt to get their money back without having to go through the issue of asking for a refund, getting denied and then having to wait 10 days for paypal appeals process. This also does not take into account that perhaps these folks paid with bitcoin and might never see their funds again.