I've got a project coming up with a lot of exterior framing - covered porches and screen porches. The rest of the house will be douglas fir, but I'm concerned about the weather resistance of the fir for the porch posts. I'm thinking of using red cedar for those posts instead, and leaving the rafters and girts as fir, as they should stay dry. Location is in Minnesota. Experiences/Suggestions/Opinions?

Good quality old growth doug fir has some rot resistance. But not nearly as much as Red Cedar though it should be noted that here too the wood with the most rot resistance is the heartwood of older trees. (see below)
I think that fir may be a fine choice with the right detailing (dry good, wet bad)and if it is not exposed to the weather full time. It should also be noted that cedar has not nearly the strength of fir but in the case of posts which are frequently oversized for appearance this may not matter

Following is a quote from the Weyerhauser web site.

"What is the natural decay resistance of Western Red Cedar?[/B]

The excellent natural durability of Western Red Cedar heartwood is recognized throughout the world. Probably one of the most persuasive European research papers is by Bellmann. This paper is in German, and it highlights Western Red Cedar as being one of the most durable species in the marketplace. Western Red Cedar contains naturally-occurring water-soluble extractives than can leach out during service (one of the reasons why it changes to a grey colour over time). For this reason, it is suggested to use a natural oil-based clear stain to improve longevity of the exposed product.

How does the decay-resistance of Western Red Cedar vary with colour?[/B]

There is no direct correlation between Western Red Cedar colour and the extractives content of the wood. Rather, it is the radial location from where the board came from that typically signifies how resistant the wood could be. Therefore, darker cedar boards do not indicate that they could last longer in service. The white-coloured sapwood contains considerably fewer extractives, but can easily be distinguished, even beside the lightest coloured heartwood.

Second growth cedar also has a lower content of extractives. Research shows that it is not a result of second growth cedar "becoming a new kind of tree". It has been observed that Western Red Cedar trees accumulate more decay-resistant extractives as they get older (and In the case of second growth trees, the accumulation is at a faster rate). As of yet, there is no forest policy to extend the rotation for second growth Western Red Cedar trees in order to increase the extractives content in the wood. This policy may have to be reviewed in 30-50 years as second growth western red cedar begins to enter the market."

It ain't what you got...it's what you do with it.

I agree with John about choosing the timber carefully, the same is true for fir: go for high summerwood content and nice, even growth, avoid the sapwood and pith. Both WRC and D-fir can be used effectively outside and there's plenty of historic precedents to justify this...but both can deteriorate pretty quickly in certain situations too. Even the sweetest cedar frame will eventually rot away if water can dribble down the knee braces and stay trapped inside joints, or if the post bottoms can wick moisture up from unsealed concrete. Longevity is dependant upon: good material selection, good detailing (especially around post bases and eaves) and a little basic maintenance. Specifying the right species isn't enough on it's own...and every exterior frame needs to be looked after a bit. Whichever species you decide to go with, ensure that the timbers can always shed water, 'breath' (good air circulation all around), and dry out after a soaking. Likewise, make sure that the proud owners know to keep your hard work free of leaf/tree debris or anything else that might trap moisture.