If its your first SLR, you have got to consider the following options.
1. Are you going to be taking a course in SLR photography?
2. Do you want something you can use right away, or something more difficult that you will learn more with over time? Is this something you want to one day seriously expand upon?
3. Is size a factor?
4. What sort of lenses are you after? This is an important one. You can get short range lenses, with good wide angle, or you can get twin lens kits that will give you the short range lens mentioned and a long range lens for distant shots, or you can get an in between lens to cover both, while costing much more. Its a tricky one.
I don't have much hands on experience with Pentax, so, I don't feel comfortable making a comment on them. However, in regards to other brands, I do feel very confident, so I'll weigh in.
In terms of Sony, the camera's do perform well, but suffer from two issues that detract their attractiveness. One, they tend to be gimmick laden - usually hiding beneath the mask of 'innovative'. Two, they have an expensive lens catalog. They have a range of SLT models, similar to a traditional SLR except with a faster frame rate (subverted because they don't have a fast enough focus motor to keep up) and an electronic viewfinder. They do admittedly make a good camera bang for buck wise, but I don't think you'll get enough out of them if you are thinking longer term.
Moving on then, to the big two. Nikon and Canon. I use Nikon myself, so I tend to be a little biased, but the truth be told, both brands make an excellent camera. Reading the entry level, the two models that are true entry level at the moment for each brand are the Nikon D3100 and the Canon 500D, mentioned above.
Between the two, each one has a good feel, both have HD movie if you are into that sort of thing. The 500D has a larger pixel count, but also a higher pixel density and a smaller sensor (marginally, but higher pixel density does detract away from range of ISO sensitivity). Both are an excellent camera, and the lens range of Canon is sensational, with great pricing, but I'm going to recommend in your case the D3100.
The reason for that goes back to your earlier comment, that this is your first SLR. The 500D is a fantastic camera, but it lacks something that the D3100 has. Looking at a 500D you will see an array of buttons, but, and this is the crucial part, no way to figure out what they mean short of consulting an expert or the internet.
With the D3100 you have two assists to this. One is the guide mode, located on the shooting mode wheel at the top. It will simplify your shooting for you, allowing you to choose what you are shooting via the screen on the back - For example, you choose 'Simple Mode' on the back, then 'Portrait at Night' or something similar from the wide range of options, and the camera changes its settings to get, as it sees it, the best possible shot in the conditions you've selected. In your case, this basically means faster shooting, less messing around.
The second assist is the help button on the back. Its a question mark symbol that, when pressed while you have any of the menu and settings options lit up on the back screen, will place an explanation of what exactly this setting is and what it can do. Canon do not have this. It is, basically, like having an assistance manual with you when you go shooting.
Now, this camera may sound like a camera for dummies, but don't be fooled. As you learn more, you can forget these two options more and more, and this camera will do everything that the Canon SLR will do as well, and maybe even a few extras. It isn't dumbed down, missing certain features, it just has a few on there to explain things better.
It does, however, have two detractions. One, is a lack of a secondary LCD screen on the top for settings adjustment. The other is that it has no autofocus when using particularly old film lenses. While these make a difference to me, they shouldn't make one to you, however.
One other issue is price. Finding it locally and not a grey import for less than $800 is going to be horrendously difficult. However, I'd recommend getting a local one. Not only does it keep money in Oz, it means you'll get warranty support as well. A grey import will not, unless it goes back to where you purchased it, and even then, it will have to be sent overseas.
Anyway, hope this helps. Post more questions as you get em, I've been doing this a while now, and should be able to answer them.