Thermal Considerations

In most typical applications a heatsink is not necessary, it is expected that the device will get warm to the touch when powered on, even the EVM does, so I would doubt that there is something else wrong with the board. Much of the heat generated by the part ends up being soaked into the power and ground planes of the board, so if your board is physically smaller than the EVM and thus has less copper in the planes to act as heat sinks that likely explains why your device might feel warmer than the EVM board's.

In general you should only need a heat sink if your board is going to be in an intense environment where there is no or minimal airflow and the ambient temperature is relatively high, of course the environment is very application specific, if this is a real concern than you will probably want to measure the case temperature, the case (surface of the BGA package) should not exceed 85 C or you may see operational failures or potentially permanent damage to the device if it is exceeded too much. If you want to dig into the thermal characteristics of the part the thermal resistance figures can be found in section 7.1 of the datasheet at http://focus.ti.com/lit/ds/symlink/tms320dm6446.pdf. Also a discussion on thermal characteristics of DM644x BGAs to help make sense of these numbers can be found in http://focus.ti.com/lit/an/spraae4a/spraae4a.pdf.

I think it is a bit optimistic to consider that it is possible to run a DM6467 without Heatsink.

It is possible to have it at ambiant temperature, with good copper and huge PCB area per chipset. If you plan to do an industrial product, I think you should consider putting a Heatsink on top of it, except if you have got an exceptional fan system...

You are correct, what type of thermal dissipation you need is very application dependent, including the environment and which device you are using, the DM6467 in particular is one of the hotter parts we have, particularly when it is working on HD video. In an industrial environment with a case and limited airflow a heatsink will become more of a good idea, but as shown on the EVM this is not always necessary, though I would probably consider the EVM an optimal thermal condition, open air, room temperature, and a relatively large copper heavy board. Either way you will probably want to do some thermal testing on your custom board to determine if additional thermal dissipation is necessary and how much you need.

 I'm using a DM6446.  Is it safe to say that this part runs cooler than the DM6467 and thus won't need any heat sinking?

DM6446 will typically run cooler than DM6467, and is thus less likely to need a heat sink. Of course if you really need one or not will depend on your application, particularly your application's environment. On an open board in a room temperature environment like you have with the DVEVM you probably do not, but start adding in a case, smaller PCB and other hot hardware sharing the same space and that can change.

I have only seen one case (small board design) where temperature became an issue with DM6446; in this case, the work around involved redesigning the casing to help dissipte the heat.  Of course, as Bernie suggested above, this is very application specific; as a general rule of thumb, the smaller the board, the less heat the system will be able to dissipate.  However, DM6446 is being used in some small security camera designs without the need for a heat sink.