What's new/key features (highlights)

24.6 megapixel 'Exmor' CMOS
No surprises here (Sony announced its blockbuster full frame sensor back in January). As well as offering currently the highest resolution in its class the new sensor is claimed to offer class-leading noise performance (it may have more than twice as many pixels as the Alpha 700, but the pixel pitch is actually larger; 5.9 micron vs 5.5 micron).

Once again the sensor uses on-chip column A/D processing (there are over 6000 parallel analog/digital converters on the sensor) and once again there is a two stage noise reduction system - both before and after the A/D conversion.

Dual Bionz Image Processors
Obviously Sony needed to beef up the processing with such huge amounts of data, and it did so by adding a second Bionz image processing engine. Working in parallel the dual Bionz processors allow the Alpha 900 to shoot at an impressive 5.0 frames per second despite its huge files.
Anti-Blur
Well, a lot of people said it wasn't possible, but Sony has managed to produce a sensor-shift SteadyShot image stabilization system with a full frame sensor. Its actuator is 1.5x more powerful and 1.3x faster than previous models, and is claimed to offer an up to 4 stops advantage. It's compatible with all lenses. The CCD shift is also used (in conjunction with a special anti-static coating on the sensor) to offer a dust removal feature.
New AF Sensor
The Alpha 900 features a new center dual-cross, 9 point AF system with an additional 10 (assist) sensors used when Wide AF is selected. There is an additional F2.8 sensor in the middle for higher accuracy with fast lenses. Sony is claiming improved tracking accuracy and focus speed (it's interesting to note that the AF area covered is rather squashed in the middle - perfectly fitting into a cropped sensor area, suggesting it may well find use in future models).		 High Speed shutter
Carbon fibre shutter offering 1/8000 sec top speed and 1/250th sec x-sync (1/200th if SteadyShot is on). Sony is quoting a 100,000 shutter cycle life.
Newly Designed Mirror Box
The Alpha 900 has a big mirror (full frame and 100% coverage kind of precludes anything smaller), so Sony has designed a totally new mirror box that uses a clever 'Parallel Link' system to raise the mirror (rather than simply swinging it) without the need for a considerably deeper body. The mechanism itself features a newly developed moving magnet actuator, coreless motor and magnetic catcher (to minimize bounce) - all of which help the Alpha 900 achieve its impressive 5.0 fps shooting speed.
Top info panel
Unlike the 700, the Alpha 900 has a small (and to be honest, almost redundant) info panel which displays battery life, remaining shots, aperture/shutter speed and AE compensation status, and can be used to change ISO, WB, and Drive modes too.

Optical Pentaprism
New high quality optical glass pentaprism and new high-power condenser lens offering virtually 100% coverage and 0.74x magnification. The supplied Super spherical acute mate focusing screen is user-interchangeable.

Aluminum chassis/Magnesium body
The front, back and top are made of magnesium alloy. Like the A700, the A900's buttons and levers are sealed to protect against dust and moisture (though it's worth pointing out that the camera is in no way 'waterproof' or 'splashproof').
X-Fine 3.0" LCD
The Alpha 900 has what appears to be exactly the same Xtra-Fine 3.0" LCD screen as the A700. With 922,000 (well, 640 x 480 x RGB) pixels it has a resolution of 270ppi, plus high contrast and a wide viewing angle.

Intelligent Preview
The A900 doesn't have live view, but it does offer a fairly unique 'preview' option. Press the DOF button and a 'pre capture' shot is taken and displayed on-screen as shown above. You can now play around with exposure, DRO and white balance to fine tune the settings before you take the picture. Note that you can't save the image you've already taken, which means it's going to be more useful with static subjects than anything else.
Custom Register
Rather than the Alpha 700's range of scene modes the 900 has three considerably more useful (and certainly more versatile) user-programmable memory modes. These are accessible directly from the mode dial.

Dual Storage Formats
Once again Sony has included that professional photography essential, the Memory Stick Duo slot (in addition to the arguably more useful CompactFlash slot). This vestigial slot will undoubtedly appeal to PSP touting hipsters prepared to pay over the odds for a proprietary memory format, but for the rest of us it's doomed to remain a small, narrow place to keep dust.
New lenses
The A900 is launched with two new full frame lenses, the 70-400mm F4-5.6 G SSM and this very nice Zeiss Vario-Sonnar T* 16-35mm F2.8 ZA SSM. Incidentally you can use Sony/ Minolta crop lenses (DT) on the Alpha 900 by switching on the 'crop mode' option.

New Vertical Grip
The Alpha 900 has a new optional vertical battery grip, the VG-C90AM. The grip offers a portrait shooting shutter button, two control dials (and a full set of buttons) and accepts two battery packs.