I can't think of any reason to switch unless you were unhappy with the experience, the ergonomics, or the controls. You don't have a huge or expensive commitment in lenses yet, so that's not too much of an issue - but if you're severely budget limited that can make a difference.
Canons are fine cameras, as are any other brands. The 550D would be a small step up - not much more. And with only the kit lens, you'd be much more limited in your lens range until you spent more money. Canon has moved on to the T3/600D which is a better camera, but obviously more money still.
The Sony A580 on the other hand would be a very large step up from your current camera, and is a fairly significant step on the 550D as well. Being able to move to the A580 and still have two lenses to start with right off the bat puts you in much better overall shape for versatility and ability to expand your photography into new areas. It's MUCH faster - both in focus and continuous frame rate, the focus system is better, tracks better, has more focus points, the live view system is a huge upgrade and the best live view system ever developed by a mile, and the high ISO performance is tops for APS-C, with the Sony-made 16MP sensor in the A580, Nikon D7000, and Pentax K5 consistently top ranked - and those two cameras are much more expensive. With more and better controls than your current camera, huge battery life, and gorgeous high-res LCD, it's clearly in another class from your current cam.
By the way, as far as lenses and equipment - it shouldn't be as hard to find equipment as you make out...have you shopped around for Minolta lenses to go with your Alpha? Some amazing, wonderful lenses out there in Minolta land, as good or better than a lot of the Sony stuff, and very cheap on the used market. These screw-driven lenses tend to be reliable and less prone to worry about dying motors or bad stabilization systems in the lenses, since none of that exists - it's all in the camera body. Many Sony users are not aware of the fact that Minolta autofocus lenses work exactly like new on Sony's DSLRs, and think the lens selection is a little smallish (105 current lenses for Alpha mount)...adding Minolta to the list opens you up to over 360 lenses...and some very cheap. I'd consider adding one fast prime to your collection - you can pick up a Minolta 50mm F1.7 for $70 or under...if the newer Sony options are too much.
Sony DSLR-A580 / Sony 18-250mm / Minolta 50mm F1.7 / Sigma 30mm F1.4 / Tamron 10-24mm / Tamron 200-500mm / Tamron 90mm F2.8 macro / Minolta 300mm F4 APO
Sony NEX5N / 18-55mm F3.5-5.6 / 55-210mm F4-6.3 / Pentax K adapter / Konica K/AR adapter / bunches o' Konica & Pentax lenses!