Simon, don't forget one of the other reasons it was not written down until the early 4th century is that while the Church remained under Roman persecution, it was very difficult--if not impossible--to call a proper ecumenical council to address these issues.
The fact that the Creed was not set down until the 4th century does not in any way remove from its authority or truth. The Canon of Holy Scripture as we (essentially) receive it today dates from the same period. While the authority of the Bible is sadly today under attack, it's generally not because the Canon wasn't settled until that point but rather because the Scriptures are inconvenient and old and their inspiration is attacked, etc.
Albion, I would also disagree with you that this isn't a movement to replace the Creed. While I'll admit that the exact words used suggest what you are saying (it wont be replaced or rejected), let's see what the practical implications are:
It is a stumbling block for many: There are parts of the Nicene Creed which Episcopalians do not wish to profess and it would be a salve to their "faith" if they were not required to cross their fingers and recite it or refrain from reciting it during the Eucharist. Okay, fair enough, but again if you cannot profess an element of the Nicene Creed (setting aside the filioque controversy) then you cannot profess Christianity. The solution here is to identify the people having issues, identify the issues they are having and discuss those problems with them until they are able to accept the Truth professed in the Creed.
Not necessary during the Eucharist due to the Great Thanksgiving's robust affirmation of God's work in Christ: again, referring back to the previous point, this suggests that there are statements made in the Creed, but not made in other parts of the liturgy, that people struggle to accept. Rather than trying to teach why the Creed is to be believed, the solution being presented is to no longer require those beliefs to be professed regardless of their normative value to Christians throughout time.
The use of modern creedal texts alongside the Nicene Creed might be a creative opportunity for engaging worshipers: Now this can be taken in several ways. I see it not as suggesting you would recite the Nicene Creed and then a "modern creedal text" but rather they will provide options, saying you can recite one of the traditional creeds or one of these new affirmations of faith. This is exactly what has happened in the Anglican Church of Canada when the Liturgy task force tried to replace the Apostle's Creed in Morning and Evening Prayer with a new "Affirmation of Faith" they felt more accurately spoke to modern people who might struggle with the Apostle's Creed. It was thoroughly rejected by the people and they dropped it from their revisions, but again I see this as being part of the same process. You list an "alternative" affirmation of face and then it becomes normative for many despite the fact that it no longer speaks to the fullness of the Christian faith. Assuming TEC is still around in a few generations, what it means to be a Christian to someone in TEC is as far from traditional Christianity as unitarian universalism is from Christianity today.
So I'll admit if I hadn't made it clear already, I am not part of TEC, but on a different scale (this was the liturgy task force's own initiative and not something discussed at general synod or approved there, and it was dropped in the face of massive backlash) it's something that I'm seeing here and I worry very much what the results would be. I see it very much, to put it bluntly, as a dumbing down of the faith in order to make Christianity palatable. But that results in basically making it a secular happy-clappy feely-goody-feelyness place where you go to meet up with others, told what a wonderful person you are, that there is a God, but don't worry he loves you as you are, doesn't call you to change or offer you grace to transform you (because it would suggest you needed to change or be transformed) and that really you should just be a nice person and be good to everyone (you get to decide what being good to others means, though you can often look to the prevailing societal views for that).