The Sony SLT A33 and SLT A55 have Sony's Translucent Mirror Technology which allows for full-time continuous auto focus whether you're shooting still pictures or 1080/60i HD video. By eliminating the mechanical components of the traditional moving mirror Sony was able to make the SLT A33 and SLT A55 smaller, more compact and quieter. The Translucent Mirror Technology also allows the A33 to shoot at 7fps and the A55 to shoot at 10fps (frames per second) - shooting rates unmatched by any other DSLR in their price range. Another advantage of this technology is that the viewfinder never goes dark. On a traditional DSLR when you press the shutter release the mirror has to flip up which causes the viewfinder to go dark until the exposure is made at which time the mirror flips down.
The A33 and A55 use the Minolta Maxxum "A" lens mount introduced in 1985 so any Maxxum or after-market (Vivitar, Sigma, Tamron, Tokina) AF lens can be used on these cameras. Since the A33 and A55 have a focusing motor in the camera body these legacy lenses will auto focus as they should. Sony's version of Image Stabilization is also in the camera body so a 1985 model Maxxum AF 70-210mm f4 zoom lens will not only auto focus but will become, in effect, an IS lens. Any lens used becomes an IS lens.
Reviews:
http://www.imaging-resource.com/PRODS/AA33/AA33A.HTM
http://www.imaging-resource.com/PRODS/AA55/AA55A.HTM
http://www.digitalphotographywriter.com/2010/08/sony-slt-a55-review-translucent-mirror.html
http://www.whatdigitalcamera.com/equipment/reviews/digitalslr/128639/1/sony-alpha-slt-a33-review.html
This explains the difference between the two models:
http://www.digitalphotographywriter.com/2010/08/sony-a33-vs-a55-whats-difference.html
Purists, of course, claim that the A33 and A55 aren't "real" DSLR cameras since they lack the traditional optical viewfinder. Translucent Mirror Technology requires using an EVF - electronic viewfinder. These same purists were equally dismissive of, first, Live View on a DSLR and later video capability in a DSLR. Nikon introduced the D3000 without video capability and quickly replaced it with the D3100. Now every DSLR must have Live View and video capability to compete in the market.
Back in the days of film photography news photographers were equally resistant to abandoning their 4x5 Crown Graphics for 120 roll film cameras and later when they had to abandon their 120 cameras for 35mm cameras. Technology marches on.
Canon, of course, still uses the traditional moving mirror in their DSLR cameras. When Canon introduced their first auto focus 35mm film cameras around 1988 they abandoned the "FD" lens mount for the "EF" lens mount. In my opinion Canon has added additional confusion by introducing the EF-S lens line along with the EF lenses. Here's why: The EF-S lenses can only be used on the Canon cropped sensor DSLRs like the T3, T1i, T2i, T3i. They cannot be used on an older AF 35mm film camera or a full-frame Canon DSLR like the 5D or 1D. So someone who buys a Canon cropped sensor DSLR and 2 or 3 EF-S lenses will be looking at replacing their lenses should they ever upgrade to a full-frame Canon DSLR.
Canon - like Nikon - has their version of Image Stabilization in some but not all of their lenses. So you pay for it every time you buy a stabilized lens. With the SLT A33 and SLT A55 any lens used becomes a stabilized lens.
Its only a matter of time before Canon and Nikon introduce cameras comparable to the Sony SLT A33 and SLT A55.