I think liberals tend to get excited about things like an "Evangelical Author [who] Puts [a] Progressive Spin On Traditional Faith" because they get so frustrated with conservative Christians always righteously claiming the moral high ground by appeal to their religious beliefs. I, for one, was initially pretty excited to get a copy (autographed!) of Jim Wallis's book. I didn't get very far through it, though, before I came to the conclusion that putting a lot of stock in progressive Evangelicals is probably a mistake for at least two reasons.
First, as a practical issue, it's not obvious to me that you can entirely divorce any sort of progressive Evagelicalism from its basically misogynist roots in the Christian tradition. It's not just that Christian history consists of many centuries of misogynist interpretation of scripture and text, it's also that much of that raw material is basically misogynist to begin with. How one might go about persuasively chipping away at the more unenlightened aspects of the Bible isn't clear. At the same time, though, an ethical framework built up from the Bible as-is just won't be suited to supporting a thoroughly progressive outlook.
Second, I think secular liberals who flaunt their newfound Christian allies over-enthusiastically implicitly concede too much of the metaethical argument to their religious opponents. Much of the tension between the religious right and the secular left derives from the suggestion that secularists aren't just immoral, but are actually fundamentally amoral, at least ideologically. Much of the force of the charge that liberals are "Godless" derives from the underlying assumption that without God you just can't make meaningful moral statements about the world.
Needless to say, I think this is very wrong and I think that liberals should be careful not to indulge arguments or impulses to that effect.
The fact is that God just doesn't do any metaethical work. If you follow any given ethical guideline in the way you live your life, God enters into your decision-making process either 1) because you think God wants you to follow the ethical guideline - in which case you still have to explain why God's preferences possess any normative significance, presumably by appeal to some metaethical principle prior to God - or 2) because the ethical guideline itself is in some way prior to God, and God just sort of enforces the rules - in which case you don't need God to have morality. In either case, ethics stand alone, without the need for any religious support.
There's nothing weird about propositions that are both secular and normative. Nothing, at any rate, that makes them weirder than, say, abstract mathematical propositions or, for that matter, metaphysical propositions. More to the point, though, if atheists are doomed to moral skepticism without God, religious individuals are no better off with Him. It concedes too much to the opposition to suggest otherwise, and it undermines the relative appeal of secular liberal ideology.
Is it better, on balance, to have Evangelicals emphasizing poverty and the environment instead of gay marriage and abortion? Probably. I just wouldn't throw my lot in with progressive Evangelicals without clearly delineating the point at which my sympathy for their project ends.