What's new / changed (key points)

15.1 megapixel CMOS sensor
The EOS 50D is Canon's highest pixel count APS-C sensor to date. The manufacturer is claiming that new manufacturing processes, plus redesigned photo diodes and micro lenses, extend the light gathering capabilities of the sensor. They obviously feel confident as they've expanded the ISO sensitivity range to 12,800 (100-3200 standard range).
DIGIC 4 image processor, 14-bit ADC
The EOS 50D is the first Canon SLR to get the new DIGIC 4 processor, which is fast enough to push 15MP images onto the card at a rate of up to 6.3 fps. With a UDMA card Canon is claiming an improvement to the buffering allowing up to 90 JPEGs in a single burst. DIGIC 4 also provides the cool new interface.
Gapless Micro lenses
Apparently the 50D's sensor has what are effectively gapless micro lenses, which significantly increases the efficiency of each pixel. Click on the thumbnail above to see the EOS 40D compared to the EOS 50D.
Expanded Live view functionality
with contrast-detect AF

No surprises to see that the EOS 50D gets contrast-detect AF (and, naturally, face detect AF).
EOS Integrated Cleaning System
Now standard across the entire EOS range, Canon is claiming an improved system for the EOS 50D thanks to a new fluorine coating.
New Clear View 3.0" LCD
An area where the EOS 40D was really starting to lag has been addressed in the EOS 50D with a new 920,000 pixel (VGA x RGB) screen that from our initial comparisons seems to be even better than the one found on the Nikon D3 (by virtue of its lower reflectance and wider viewing angle).
Quick Control screen
Building on the info panel seen on the EOS 40D; you can now control everything on the screen using the joystick controller (taking a leaf out of Olympus and Sony's book).
Creative Auto Mode
A new variant on program mode takes care of focus and exposure and uses simple sliders to control background sharpness (depth of field) and AE compensation.
Peripheral Illumination Correction
Per-lens vignetting correction pre-programmed with a large database of Canon lenses.
4 levels of High ISO Noise Reduction
The EOS 50D is the first Canon DSLR to get four levels of high ISO noise reduction (including OFF).
Extra small sRAW option
As if the EOS 40D didn't offer you enough file size and quality options, the EOS 50D throws in another RAW option (so you can now capture RAW files at 15.1MP, 7.1MP or 3.8MP).
Refreshed user interface
Courtesy of DIGIC 4, the EOS 50D's interface has not only been given a stylish redesign; it's now got some very cool 'fades' between menu items.
Better environmental sealing
Apparently (we've been told verbally) the 50D's body offers better environmental sealing than the EOS 40D thanks to a slightly different construction and tighter seals.
HDMI output
New for the EOS 50D is HDMI output for use with compatible HDTV displays.
AF Microadjustment
The 50D now offers the AF microadjustment feature seen on high end EOS models. You can adjust all lenses by the same amount or up to 20 lenses individually (+/- 20 steps in each case).