I don't think you're going to see massive selling of coins once ASIC's arrive.
Here's my thoughts:
1. Most people buying large amounts of ASIC's were existing large GPU miners. They bought with BTC (from BFL) so they already had the capital and likely it was profit from their existing farms. I don't think these folks are short-sighted and therefore will likely sell incrementally as before. If price is too low they will hoard.
Of course no one individual knows for sure what the other miners will do. The threat of one miner with an ASIC dumping is so great that it most likely will temp SOMEONE to dump. If only the regular people who fear big miners dumping at their expense.
2. Smaller buyers don't have huge capital investments, so again they are likely to hoard if prices are too low in hopes of getting a bigger payoff down the road.
Smaller buyers might also be more short-sighted if they can't/won't invest large sums of cash. In which case they want their ROI as short as possible. Dump while prices are high, recoup your investment and then hoard. If people have trouble parting with a couple hundred dollars, they will have trouble holding onto their BTCs
3. Most of the larger miners bought early, so they are also likely the first to receive their units. Initial difficulty rise will be propagated by these miners and they will be less likely to sell then those who receive their units later.
So did a lot of non-miners and small-time miners. Not to mention that the other ASIC developers aren't nearly as large as BFL and if they are one person operations might be more tempted then others to test-mine their equipment as soon as they get it working AND dump at the same time to recoup their investment of time and money.
4. Overall less coins being mined = lower supply at a base level. So let's say half of all newly mined coins get sold currently(which I doubt) then there will be no more coins sold than usual since half the amount of coins will be mined. This would be if EVERYONE sells the newly mined coins immediately.
Not everyone has to sell newly mined coins. It just needs to be enough people selling newly mined coins to trigger existing holders to panic: either to dump and get out, or dump with the hopes of getting in lower. The threshold is very small I think.