Update, 7 September 2010: According to EOSHD.com, Ichiro Takagi, Deputy President of Sony's Imaging Business Group, announced that Sony will be adding manual video controls to some of its NEX- and Alpha-series DSLRs (most likely the NEX-5 and Alpha A55, although Takagi-san didn't mention specific models.) The new firmware is to be formally announced at Photokina at the end of the month, so unless Sony announces yet more new models between now and then, the firmware will upgrade existing models. (Now, if Nikon will only get on board...)Nikon and Sony have recently announced two important DSLRs: The D3100 from Nikon and the A55 from Sony. Both cameras have 1080P (at least under some conditions), both have continuous autofocus in video mode, and both are priced very aggressively (especially the D3100). However, both are missing something every important: Manual controls in video mode.They do video in full auto only.
Both cameras support both manual and full auto operation in photo mode, so all the capabilities for supporting manual video are built in. Everything, that is, except for manual support in the cameras' firmware. Why did Nikon and Sony choose not to implement manual video mode? One argument is that given the target audience for these cameras, first-time DSLR owners moving up from point-and-shoots, manual video would be too difficult to use and too intimidating. However, shooting good pictures in manual still mode isn't easy, yet both cameras can do it.
In my opinion, the reason reason why Nikon and Sony have left out manual mode is that they don't want to cannibalize other, more-expensive products. Nikon is likely to have a higher-end DSLR with manual video mode coming very soon for $1,000 or more (U.S.), and to date, Sony's position on full manual is that if you want that, you'll have to buy one of their camcorders. Only Canon has gotten it right, with 1080P and full manual control in the T2i/550. Canon, which sells at least as many DSLRs as Nikon and quite a few more than Sony, trusts its customers to be able to flip the necessary switch to enable or disable manual control.
Manufacturers have the right to put whatever features they want into their DSLRs. However, if a customer wants manual video control, they're not going to accept full auto, and if they can't find manual in a camera in one product line at a price they can afford, they'll buy a camera from someone else.