There are several reasons why Dish does not "support" 1TB drives, including the vertical recording techniques used to get to 1TB. The question is, when your 750GB drive fails in some way, what support do you really expect to get from Dish? If they answer is "none" (which is the most correct answer), then all your decision really amounts to is whether you have another use for a 1TB drive if it does not work with your DVR. Most people would say that they do have another use.
People posting on other forums have reported getting 1TB drives to work with no issues. I personally got a nice quiet 750GB drive as I have friends in the storage business who say don't use 1TB drives without a really good filesystem, like ZFS, otherwise you are likely to lose some data at some point. For the record, Dish does not use ZFS as their filesystem. Last time I checked, it was XFS. I guess I just assume Dish's implementation of an open source technology from a company (SGI) who would be bankrupt without the federal government's involvement doesn't make me feel very secure about my data to begin with. YMMV.