Paraphrasing you, I thought you said:
A: From the observation that the chances of life are (exceedingly) small
B: it is valid to conclude there is a designer.
All I said is that B is not a necessary result of A. That's what I think "still a perfectly valid conclusion" means.
However, you may have meant that B is not *ruled out* by A. That I agree with. That life was designed is not ruled out by observing that the chance of life occurring (when we take the universe as a random system) is small. But starting from A it is not valid to definitively conclude B. A does not imply that B is true.
A is consistent with both conclusions, that there is no designer or that there is.
Intelligent Design proponents set up these probabilistic arguments to show that the "probability" of evolution being true is small. Then they argue from ignorance, saying, "because I can't think of any other explanation for life, given that it seems exceedingly unlikely that life evolved on its own, then it must be true that there is a designer". There could be some other explanation for life that we haven't thought of yet. No one has proved that there are only two choices. So, even if someone proves that evolution is definitively not the answer (with probability 1.0), we still can't conclude that a designer is the answer.
Calculations like these are what drive science. First of all, we know they are wrong to begin with. We are trying to capture an immensely complex process by a few numbers and a very limited kind of structure (multiplying probabilities). Therefore, arriving at a very small probability for live evolving by chance raises questions. Are the probabilities right? Are there conditional probabilities that we haven't taken into account? Is the process really random like we are assuming? Are our other assumptions correct? Even if we convince ourselves that we're in the ballpark, a small probability may be surprising, but that doesn't make it wrong.
A more scientific kind of reaction to this is the anthropic principle: http://en.wikipedia.org/wiki/A.... The money quote for me is the weak anthropic principle, "which states that the universe's ostensible fine tuning is the result of selection bias: i.e., only in a universe capable of eventually supporting life will there be living beings capable of observing and reflecting upon any such fine tuning, while a universe less compatible with life will go unbeheld."
Thus, given that the calculated chance of life evolving is small, one alternative to a designer is the idea of parallel universes (i.e. a multiverse: http://en.wikipedia.org/wiki/M...).