Cake flour is a low protein, low-gluten and fine-milled bleached soft wheat flour. Unbleached pasta flour found in European Union countries isn't a true substitute for North American cake flour; the chlorination process changes the protein structure in wheat flour, and improves the way sugar and fats bind in their magical way when you bake. The results are more interesting than the chemistry: a cake made with bleached cake flour will have a finer texture and rise a bit higher than any product made with finely milled - but unbleached - product.
If you live in the United States or Canada, it's easy to find bleached cake flour; look in a large grocery for Pillsbury's Softasilk® or Presto's Swans Down® brands. But if you are a North American ex-pat trying to bake a tender torte using a recipe that calls for American cake flour, there's a way to recreate unbleached pasta flour to bake those sweet and sinful creations in your grandmother's recipe box: try Kate flour. Kate flour is made from unbleached pastry flour, and her process involves microwaving, cooling, sifting and rehydrating in the oven with a water bath. The final step is whisking in a small amount of xanthan gum into the treated flour. The detailed process, including tips on using flour found in countries throughout the EU, is described at Kate's blog, A Merrier World. I'd describe her full process in this write-up, but Kate's description is best left un-paraphrased these days, and the tips readers have left for local flour are quite valuable.
I rarely bake cakes, and I'll never try bleaching and rehydrating unbleached cake flour as long as I can buy it at my local market. It's your call if that makes me an ugly American or a sensible woman - but the process has been tested by pastry chef and cookbook author extraordinaire, Rose Levy Beranbaum - I'd follow her cakes anywhere.