The difference, I believe, is mainly in the specific processors they use. The Timeline has a dual-core ULV, meaning there are two computing engines, while the X-Slim has a single core. The single-core ULV appears to be only a slight step up from the Atom — something to keep in mind when looking at other models that are sure to come out with these processors. (There's a confounding factor here: The Timeline runs the 64-bit version of Windows Vista, which processes data in larger chunks, while the X-Slim runs the somewhat slower 32-bit. But the differences I observed were too large to be attributed to the software.)
The Timeline slays the X-Slim in battery life too. I made each computer play high-definition video from the hard drive while running a Twitter application that accessed the Internet over Wi-Fi. That ran the X-Slim down in one hour and 42 minutes, while the Timeline lasted three hours and 40 minutes. That lends credence to Acer's claim that under a typical workload, the Timeline will last more than eight hours.
For comparison, the Asus Eee PC 1000HE, an outstanding netbook, lasted four hours and 45 minutes. (All screens were set to the same midlevel brightness, measured with a light meter.)
So I have a mixed verdict on ULVs. The Timeline at least shows that the processors can be used in laptops that are reasonably powerful.
Advanced Micro Devices, Intel's main competitor, has launched a set of chips it calls "Neo" this year, for computers in a similar price range and size. The first computer with these chips, the Hewlett-Packard dv2, is heavier than the ones I tested and has a battery life more in line with the X-Slim than the Timeline. But AMD plans better processors soon, and they could provide an interesting alternative.
The Timeline has a couple of knocks against it: Its cooling fan is quite loud, and the screen doesn't bend back very far, which can be a problem if you like to use it while curled up on the couch. Also, it's still a bit expensive at $900. But overall, it's a light, long-lasting computer that avoids the unnecessary design flourishes that compromise the X-Slim.