Yep, it does depend on companies not sharing serial numbers between unrelated product lines. Intuitively this feels unlikely to be the case but I guess it is possible.
We do have some data (e.g. from the map) which will help us figure out if the predictions are likely to be right, e.g. relative frequencies of 454s, Solexas and SOLiDs in the wild.
My boss always says "Why think, why not do the experiment?" - lets have a go and see if it works! Please send your numbers in!