I was looking at the Teched sessions and one of the aspects that cut my interest was the limit for application download using 3G network (maximum 20MB). It's a choice that Apple made and from my experience with the end user they are loosing clients for some applications. For example A friend of mine that owns an iPhone, uses it, loves it but is not able to connect it to a PC via iTunes. It's not said that somebody that will own a WP7 will also know how to connect it to a PC (it's a sad reality but it's like this). A lot of people will buy a "trendy" phone but don't know how to use a PC, or more are not interested in connecting the phone to the pc.
Another scenario would be that I am in a town, don't have a car navigator but I need to arrive in some place in town and I don't have an internet connection besides the one my WP7. In this case I would happily pay 40Euro to download TomTom with Italy map and I would wait 5-10 minutes to download it (download size around 400MB).
So why "block" the user buy applications that they are interested/need it? The price in Italy for unlimited data traffic it's really small (Tim Italy offers unlimited unlimited data transfer from for just 2 Euro/week but only from the phone)? Ericsson declared that the world wide data traffic over the cellular network surpassed the voice traffic.
If battery it's a problem wait for the user to connect the power plug. I personally would prefer not to power up my pc, connect the phone and download the application especially if it doesn't matter if I download using 3G or ADSL. You are also cutting off the users that don't have internet at home but have the latest phone on the market in their pockets and those are the users for which the price doesn't really matter.
The only reason I can think of is if the voice provider Vodafone, T-Mobile is imposing a maximum download size (it would be strange, but also in that case I cannot believe that all the providers have the same conditions).
So my way would be WARN THE USER but if he agrees DON'T LIMIT HIM