I work at a large automotive supply corporation in the States, and we have a large system in place to deal with this sort of issue. Here, cars are designed to last for at least 10 years, and an additional problem is that the development, manufacturing, emissions certification etc. takes a lot of time, money, and effort.
Therefore, our ordering department runs every part through a thorough approval process, which attempts to guarantee that a design based on the part will not be negatively impacted by glitch in the supply chain. For instance, we use a Texas Instruments TLC271 programmable gain op amp in some designs. We recently had a problem with this part's stocking, so we switched to the ST micro equivalent. We could also have switched to a number of other parts.
This isn't such a big deal with analog parts - Everyone makes 74XX, 1n4001, 2N700, 2N3904, etc., and finding one at a different supplier isn't too hard. However, it's more difficult to do this with microcontrollers, because the code isn't usually portable from, say, a PIC to an AVR, without a LOT of work. If Atmel went out of business, and you used their UC3 chip, you'd be in a bad spot. Your entire product line would be gone. A LOT of people would also be in trouble, and it's unlikely, but it could happen, and almost did a while back (Might have been Microchip, can't remember). This means you want a microcontroller which is produced by several chip manufacturers - in your case, you want an ARM device. The UC3's peripherals are nice, but have you considered how nice it would be to watch the UC3 go out of stock, and, while your competitors flounder, you simply purchase an ST or TI part instead, solder it on, recompile (you'll probably need to make a few minor adjustments, the system isn't perfect), and continue to ship your products?
If you're looking for a small DH, make sure that they plan for these possibilities. Be wary of a design house with a 'favorite' manufacturer. You'll likely have more sucess in this department with a larger DH that can afford to have one or more people dedicated to selecting parts, rather than depending on the engineers to make a good selection and select components with redundant supply chains, but I won't deny that there are a lot of nice things about a smaller company.
