The Vostro 1400 is a better bet than the 1500.
How long will it run with no fan? I'll be able to post in a month or so as I plan to buy one, but it should outperform every other new Dell I've been able to find specs on.
Most of my digging has been based on working through the manuals to see how many components the main fan cools in each model. If it cools the GPU, chipsets, etc you can bet that model won't run passively for very long.
Secondly, the 1400 is more volumeous than most notebooks these days, allowing more internal air circulation (there are plenty of air vents).
Thirdly, it boosts lower CPU temps than the T61 according to a forum thread I fould. Achieving this despite having Dell's lowest stated noise level for any of their notebooks (by 1db at 50cm) spells good news to me.
I'm currently using a T41 which will run passively all day long (with low CPU utilization) but which had some weird compatibility problems with the Transend SSD I bought for it (and returned).