The first generation Sienna was sold from 1998 until 2003. It was a front wheel drive minivan, unlike the less conventional Previa that it replaced. Toyota did not hide the fact that the Sienna is based on the Camry platform. They even advertised it as the Camry of Minivans. Fuel economy on the Sienna was 18 city, 24 highway.
A minor update was made to the first generation Sienna in 2001. It was given some styling tweaks, some new features like side impact airbags were added, and a new VVT-i engine was added to the options.
The second generation Sienna has been sold since 2004. It has a new engine, an option for all wheel drive, and slightly better fuel economy. A navigation system and rear camera system are also a new option. The XLE Limited (now just the Limited) was added as a trim option. Dual power sliding doors are offered on that particular trim.
The current trim lines include the CE, LE, LE AWD, XLE, XLE AWD, Limited, and Limited AWD. Prices range from $24,000 to about $37,500.
The next generation Sienna will probably come out in 2009 or 2010 and be based on the FT-MV, which was a concept car that Toyota introduced in 2007.
The Sienna can be found on the used minivan market. If you are looking to buy a Toyota Sienna, don't just check KBB for the current value. Check eBay listings to see what's out there and what it's selling for.