I think you're hit the nail on the head. To be fair to ASUS, the Transformer (which I own) is not that expensive, but it does cost around what a high end smart phone does.
I also own a Galaxy S2. Those are not cheap either.
I imagine that because they're likely to sell them as a package, they're not be as expensive as buying a phone and Transformer has for me (fingers crossed :)))
The price point is going to have to be good to attract users.
I've used iPads (friends have them) and I really can't say that you can choose between a Transformer tablet and an iPad. Reviews say the same thing. If you use a lot of iToy ;)) things then you're going to want to keep using them because they're designed to work together and keep people within that shiny, shiny, shiny (did I say 'shiny') white mirrored room. However, on a purely capacity/hardware/function basis, there's no difference.
So, the phone will need to be very good to cause people to want to buy the whole device. I would get one if the phone was as good (or nearly) as my Galaxy S2 and ran Android ('natch'). However, if the phone is mediocre, then I'd be sticking with my current Transformer/phone combination.
I think ASUS are going to have to really work hard on the phone because the Transformers show they can already make great tablets.