I would like to know why we can only save a document as PDF v1.4, while in Illustrator CS3 we can save in PDF format with a compatibility up to v8 Did i misunderstood something about PDF Cairo or inkscape isn't up to date ?
The very latest version of Illustrator can save a PDF compatiable with Acrobat v8, but the PDF version for this is 1.7. So it's not like it's 7 full version ahead or anything. And if you've used any of Adobe's product you'll notice by default is selects PDF version 1.5 to save your PDF's as. This is because 1.5 has higher compatiability and most documents don't need any of the features in more recent versions.
Also, the PDF standard is created by Adobe. It's in their best interest to include the newest standards in all their products.
If you really want to know why Cairo support capabilities similar to 1.4 you might like to ask on a Cairo forum, but I dare say that the newer features in 1.5 aren't needed for most uses.
I believe some of the features of 1.6 and 1.7 are things like creating forms, or adding notes to a PDF, or workflow features--the kind of features that Inkscape is unlikely to ever need as a drawing program.