Sony makes good stuff, in fact in all of your pro and consumer nikon cameras (with the exception of the new D3), are all sony sensors. Virtually EVERY point and shoot camera on the market (including canon) are sony sensors. So they know what they are doing in that regard. Also, Sony actually has a pretty impressive line up of lenses and accessories, which will also continue to grow as they get a larger and larger piece of the photo market. You can defenitly confidently invest in Sony. Also a lot of their lenses are Zeiss approved which means HIGH quality. Sony is defenitly giving canon and nikon a run for their money. Here's a fun little story...
Before the D3 was announced we had the canon rep come in and he was saying how canon is the only camera company on the market that builds their own sensor, processor, etc. My co-worker Chris said, "well, Sony does too."
Canon rep "yea, but their not a real camera company"
chris "but they will be..."
Yesterday, the Nikon rep came in and admitted that the biggest threat to them wasn't canon, its Sony.
The only down side to investing in Sony is that they tend to be VERY proprietary, meaning frequently you will NOT find off brand accessories, but then I come back to, they have an impressive lineup of accessories. So what does that mean to you down the road? That when you do buy accessories you'll spend a little extra $$ because you'll have to buy sony and not promaster.
ALSO, because Minolta was absorbed by sony ALL your old Minolta lenses will work on the Alpha. So even if you don't have any, you can buy old used minolta glass and save a few pennies. Going back to what I said earlier, you can confidentally invest in Sony, they are only going to get bigger from here.