Not sure what you mean by double spend... Would you care to enlighten me please? Or should I go do my own research? Hehe not sure how newbie friendly this community is... Don't want to start off asking stupid questions but...
The reason for waiting on confirmations is protection against an attack known as a double spend.
A double spend works like this:
1) I spent some bitcoins
2) I then immediately somewhere else spend same bitcoins to another merchant.
3) Only one of those spends will be included in the next block.
If you are a victim of a double spend you have a 50% chance of getting coins. Luck of the draw. Once a transaction is in a block that is 1 confirmation. The ony way that could be undone is a theoretical attack known as a "51% attack" which is very expensive.
After 6 confirms the likelihood that all 6 blocks could be undone by a longer chain in an extended 51% attack is very remote.
So 0 confirms = risk
1 confirms = much lower risk
6 confirms = essentially no risk
Now I just said 0 confirms is risky so how could a business take 0 confirm transactions? Well if you are moving $20M in irrevocable bearer bonds you better not skip waiting for 6 (or maybe 20) confirms.
However if it is access to a porn website (or a $3 burger) the risk of a double spend is very small and the potential loss to the business is very little. There really is no need to wait for 1+ confirms. No business need to avoid 100% of fraud just keep fraud at a low level. CC, checks and cash are still used despite billions of dollars a year in fraud.
Many people think I must have 6 confirms or "ultra risky". Just pointing out that risk is all relative. 0 confirms is lkely less risky (based on what we have observed so far) than accepting a credit card for small transactions.
