2 very important questions and anserw by Sunny:

questions:
1. from wikipedia http://en.wikipedia.org/wiki/Primecoin "To meet this requirement, the size of the prime numbers in the system cannot be too large". What could happen when we cross 10 or 100 mln block and the prime numbers start to be large?
2. "If the coin is successful and prime number theorists can use the results to find an equation to predict prime #s, it would essentially break the coin's PoW." -> is it possible?

answers:
1) Prime size would stay same. The prime chains aren't going to be exhausted. There are countless prime numbers, basically as abundant as numbers themselves. Remember there is a prime every couple of hundred numbers at the scale primecoin miners work. So for a 256 bit number there are more than 2^248 primes, so that's not far from 2^256 which is the total numbers at 256 bit.
2) Prime number theory is an extreme hard discipline, with hundreds of years of history. So it's not very likely to happen overnight. I'd say even if you throw trillions dollars at such research it's not going to produce that type of results on demand. Also, even if you find a super efficient mining algorithm doesn't mean you 'break' the proof-of-work system, it just means you have an advantage over the public before you publish the findings. The difficulty can scale enormously in primecoin's design.
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its quite optimistic. those 2 questions were my major doubts about prime. now i think that this model of PoW could be more safe then SHA256, which could have some problem when quantum computer appear on the market.