I have just returned from roughly four thousand kilometers (roughly 2500 miles) of towing a 3000 lb Boler
B1700 around British Columbia (and back and forth to the Edmonton area) with my 2004 Sienna LE. Braking is fine (I have the stock brakes on the trailer and a common controller; the LE has all-disk brakes) and I had no problem with keeping up with traffic, or controlling the trailer.
My new discoveries on this trip included Highway 99 (the whole thing, Cache Creek to Vancouver), which is one big series of curves, hills, and rough pavement - good fun even with the trailer attached. The looong grades of Highway 97C and the Coquihalla can be tedious, and the fuel consumption looks scary on the way up, but the downhill averages out the fuel issue and I still kept up as fast a pace as any other RV or commercial truck traffic overall.
Traction going uphill on loose gravel can be an issue - I have to be careful to minimize wheelspin in some situations, typically pulling in or out of campsites with steep approaches. The traction control of the higher (above LE) trim levels would help, as of course would AWD.
I use more fuel than Alistair, but my trailer is heavier, wider, and taller... and we carry too much stuff in the van as well.
The 2007 version is different only in the engine. The new one is certainly preferable, simply because it is more powerful at every speed, but I find the 3.3L more than adequate.
For more than you probably ever wanted to know about towing with this specific vehicle, I suggest checking out the Towing subforum in the SiennaClug.org