Remember, the town assesses your property as of a particular date (like January 1st, 2008). To be fair, everyone is assessed as of the same date. Some places reassess every house each year, others go on a rotating schedule (like every 3 years), but make general assumptions about the value of a house in between true assessments (like if all houses dropped 10% in value, they just drop the value 10%). Often, since the assessment process takes time, they may not reassess your house for as long as a year and a half after the date you were assessed as. (So, lets say they go by calendar year, assessing houses as of January 1st, 2008, it may take them to July to finish, the next reassessment will be as of January 1st, 2009, but you might not get reassessed until July, 2009, etc)
If your assessed value is significantly higher because of the fact its assessed at an earlier date when prices were higher, they are unlikely to change it. However, if it is on a rotating schedule and hasn't had a close look taken at it in a while (and has lost more value than similar houses) or it has been recently reassessed and you feel its still higher than others, you can usually make an argument for lowering it - but you'd have to show good reason why you think your house is overassessed - as far as I know, my town won't "just reassess' - you can go to them and say 'These houses are comparable to mine and are assessed lower, so I should be assessed lower'. Usually you have to make the case. You can get your assessment lowered if you have a good argument, I know folks who have in my town.