Mike - to be fair, it is no more arrogant to assume that there is no other intelligent life, as it is to assume that there *is* other intelligent life out there.
For a start, on the *assumption* that life has developed on more than one planet, then one planet (and one planet only) has to come first. And so far, there is absolutely no evidence based reason why that planet shouldn't be earth. Perhaps the conditions for life to progress beyond a single cell-type nature really are so specific that it has only happened once? Just look at the Earth - if you took a sample from the atacama desert or the poles, then you would be truly hard pressed to find any life at all, let only advanced and intellligent.
It is a lovely thought that we aren't alone, but I feel it is arrogant to assume one way or another, until definitive evidence is found to the contrary.
Oh, and another thing, as our sun is also known as 'sol', then there really is only one SOLar system
(a friendly lexicographical nit-pik!)
Cheers,
Adam