The more bitcoin is used for the purpose it's supposed to be used for, the greater the demand. If more people want bitcoins to spend, they have to get them first. At a guess there'll be a delay of 12 - 24 hours until the coins get back into the system. When more people use bitcoin to purchase items, more coins will be unavailable for a short period of time.
But the effect of a permanent larger number of bitcoin users is different to what is proposed here. For the Silk Road discounts this weekend, changes i bitcoin price will depend on i. whether new users are attracted to bitcoin because of the discounts and ii. if they will become permanent bitcoin users (and iii. whether we think the fickle minds of speculators and those with enough buying power to influence the market by themselves will be trying to derail the bitcoin price change).
If the answer to i. is 'no', then there might be a short term increase in coin price as demand increases. But after the sale there'll be no percentage increase in the number of coins permanently out of circulation.
If the answer to i. if 'yes' but the answer to ii. is 'no', then the btc price increase is likely to be higher as demand increases due to even more short term demand, but again after the sale there'll be no percentage increase in the number of coins permanently out of circulation and I'd expect the price to return to a similar level to pre sale.
If the answer to i. if 'yes' and the answer to ii. is 'yes' then I'd expect the same btc price increase during the sale, and a correction to slightly higher btc prices than current.
All of this assumes the answer to iii. is 'no' which is unlikely. Any of these price increases could start a rally, followed by a correction, followed by ummmm no idea. But long term, an increase in bitcoin users will lead to an increase in the percentage of bitcoins permanently out of circulation and increase price.
Consider the US dollar. You lose money by holding US dollars because they do not yield interest like treasury bills. However, you can't hold treasury bills and trade them for dollars whenever you make a purchase. This would be too costly in terms of time and fees. To balance these two problems, you hold enough dollars to satisfy your near term spending needs. It is this demand that makes dollars valuable. The same idea applies to bitcoin.
Everyone can hold bitcoin only temporarily. However, even demand just to hold for a short while can increase price.