Is the smoke definatly white? or does it have a blue look?
If its just white and you are having no other issues such as missfire, loosing coolant, oil level dropping etc then its more than likely normal exhaust steam.
You see when an exhaust starts to cool (i.e you sat in traffic) the gases that have been converted from the cat may start condense (Some of the gases are converted are from the oxidation of unburnt hydrocarbons which create carbon dioxide and water) on a cooler day you will then see steam coming from your exhaust tip as the gas hits the cold air condenses to form steam.
Sometimes it can seem quite severe (depending on weather conditions usualy - colder the more steam you will see) there are a few threads on here about owners that are worried about this and all its turned out to be the above.
If its blue and you have checked eveything like you have in your post then it indicates worn valve stem seals, or potential piston ring failuire.