The Digital Rebel T1i uses a 9-point wide area AF sensor with the points set in a diamond pattern, similar to that of the Rebel Xsi and EOS 50D. The focus point can be manually selected, or the camera can decide which AF point(s) to use. There are 4 focus modes: One-Shot AF, AI Focus AF, AI Servo AF, and Manual. The Rebel XSi has a cross-type (dual axis) AF sensor at the center that's effective with all EF and EF-S lenses and which provides enhanced precision with lenses having maximum apertures of f/2.8 or faster The center cross-type AF sensor reads a wider variety of subject matter than conventional single-axis AF sensors and thus increases the new Rebel XTi's ability to autofocus quickly and accurately. The outer 8 AF zones use linear (single axis) sensors (which differs from the 40D/50D where all the AF sensors are cross type).
AF is generally fast (especially with ring USM lenses) and accurate. The small viewfinder size doesn't help much when using manual focus, but for most users that probably won't be an issue.
The resolution of the Digital Rebel T1i matches that of the EOS 50D, just as you would expect since they have the same sized sensor with the same number of pixels. Though going from 12MP to 15MP (XSi to T1i) or 10MP to 15MP (EOS 40D to T1i) sounds like a big difference, the increase in actual obsreved resolution is actually quite small and probably wouldn't be noticed in small prints. In theory going from 10MP to 15MP should give you about a 22% gain in linear resolution, based on the Nyquist sampling criterion and if all other variables were equal (such as the effect of the anti-aliasing filter, the RAW file demosaicing algorithms and assuming the lens being used didn't limit resolution).
The image below shows the difference observed at ISO 100, using an EF 50/1.8 lens at an aperture of f5.6
The Digital Rebel T1i image is shown at 100% in the image above. The 40D image was upsized to match the T1i image. As you can see the difference is pretty small, but the T1i does have a slight edge.
On the subject of resolution, I've seen the statement repeated a number of times that in order to take advantage of the higher resolution of sensors such as the 15MP sensor in the Digital Rebel t1i you need to use the best "L" series lenses, and maybe you even need to use them at their best aperture, in order to see the higher resolution. Well, it turns out this is just not true.
I tested the 40D against the Rebel T1i using the worst lens I had available, the original EF-S 18-55/3.5-5.6 kit lens. While it's not a bad lens by any means, it's not the sharpest lens in Canon's lineup and it's a lens that many people will chose as their first wide to normal zoom. In all the tests it was possible to detect slightly higher resolution in the T1i images.
The bottom line is that the Digital Rebel T1i resolves more image detail then the EOS 40D, and you don't need expensive "L" series primes to realize the higher resolving ability. However again I'll comment that the difference in resolution is quite small and would only likely be noticeable in large prints
The Digital Rebel T1i has the usual EOS exposure modes (Manual, Aperture Priority, Shutter Priority and a variety of Program and Auto modes) and metering patterns (Evaluative, Center Weighted, Partial and Spot).
Auto exposure bracketing with the Rebel T1i is possible over a ± 2-stop range, from -4 stops to + 4 stops. For example, what this means is that you can shoot one shot at -4 stops, one at -2 stops and one at the meter reading, or you can shoot at the meter reading, +2 stops and +4 stops, or you can shoot at -2 stops, the meter reading and +2 stops. What you can’t do automatically is bracket at -4 stops, 0 stops and +4 stops from the meter reading. You can only bracket over a ±2 stop range, but you can center the bracket over ±2 stops from the meter reading. This sounds more complex than it actually is.
Exposure accuracy is generally pretty good. There is a slight tendency to give a little more exposure than, for example, the EOS 40D as can be seen in the illustration and histograms below:
The Digital Rebel T1i seems to produce images that are about 1/3 stop brighter than those from the EOS 40D, even when the recorded exposure is the same. Normally this isn't a problem, though it could lead to a slighly higher possibility of "blowing out highlights" under some lighting conditions.