There's more to it than just the rack. Pressure cookers are different because they take longer to heat/cool than a canner. If you are following a recipe of some sort, the times are set for a canner and not a cooker. Thus you run the risk of canning unsterilized food and the possibility of food poisoning.
If your goal is canning, you should get a pressure canner. If you want a pressure cooker but would occasionally use it for canning, then get a small one (so it heats up and cools down faster).
So ultimately you won't be able to rely on standard canning times with a pressure cooker. Does that help?

yeah, it does. Is there any easy way to translate times from canner to cooker?
I'd like to try my hand at canning, but don't want to drop that kind of dough into a canner when I might only use it a couple times.

Just wrote you a long answer and lost it, dammit.

If you want to start with canning, definitely don't start with pressure canning. It's advanced and it's riskier. Instead, you can begin with hot-water-bath canning of high-acid foods. All you need is a big pot and clean jars and lids and water. You can can tomatoes, pickles (of all kinds), chutneys, a lot of fruits, salsa, tomato sauce, and stuff like that very easily without using pressure. The acid in the foods helps them retard bacteria growth, so this is the best way to start canning as a beginner - no fear. After you get the hang of canning this way - and find out whether you like doing it - you might want to invest in a real pressure canner.

You can get a ton of good recipes and how-to's just by Googling "simple canning" or "basic canning." But your best resource is actually your extension agent...if you're in the US, which I'm not sure. Your closest agricultural university can put you in touch with their cooperative extension - one of the jobs of extension is to teach people to preserve and store food. They usually have handouts about canning, and they'll also have access to an expert who can help you translate your processing times, if that's recommended.

great answer, as always, Miko!

I thought tomatoes needed to be pressure canned since so many aren't as acidic as they used to be?
I actually wanted to can spaghetti sauce, not just tomatoes, so I figured pressure would be a safer bet.

Alternitively, is there a way to test the PH of something easily at home?

I thought tomatoes needed to be pressure canned since so many aren't as acidic as they used to be?

No, they're fine. Also, a lot of people add a tsp. of lemon juice to each jar, which totally takes care of it.

Definitely give your extension agent a call, though - the handouts they have are really great and very clear.

Ahh... good to know!
I've really liked the idea of just having jars of my own sauce on hand all the time, and there are tons of farmers markets around here. seems like a waste not to take advantage.

thanks so much!!