Fact is that even pressure treated timber will eventually rot if it is in contact with the ground, or if there are places where water gets trapped.
I'm afraid I built my raised beds out of 'tanalised' timber, which is far from eco-friendly (arsenic and copper compounds applied in a vacuum chamber - gives the timber a greenish colour), but should make things last for maybe ten years. Some people are concerned about these chemicals getting into the environment and food grown nearby, and I have to say it is not my ideal solution - I'll keep you posted if it proves fatal.
If possible, keep the timber a couple of inches or more clear of the ground - maybe build a couple of courses of bricks to mount the frame on, and design it so that water gets shed rather than trapped. But also consider some sort of preservative. If you can, soak the timber before you build the coldframe, preferably by standing the endgrain in a bucket of preservative for a few days, as well as applying it all over.
Creosote is now banned in the UK, but the stores are full of 'something similar'. However, a boron solution seems a promising alternative, with, apparently, some evidence that it is not particularly harmful. Have a look at
this page from the Green Building Store.