This post interested me:
Yeah, no. They aren't going to add a USB port in a future release. It's really amazing that so many people think the iPad should have one, when there really is no need at all.
And sure enough we find:
I'd like USB for flash drives, but Apple probably wants you to buy the increased storage iPads for more space. I would also like to attach a printer.
So I think when people say they want a "USB port", they really want the port, plus a whole heap of drivers and software behind it to cater for whatever they throw at it. No disrespect to faroZ06, apologies for using your post as an example. The point is, that the iPad doesn't offer a slew of third party supported hardware drivers. Partially that is a marketing decision, but it also a technical reason the iPad is fast and reliable. When you start adding drivers for all sorts of third party equipment, you end up with, well Windows. That's what a fully fledged OS is for.
If Apple put in a USB port, there would be a torrent of complaints that it is useless, or that it doesn't cater for this or that drive, printer, camera etc. Sure, third party developers will use it, but then you'd have an app for each and every possible device, and it would be cumbersome.
As pointed out, the dock connector IS a USB port. In fact, yes, I have a standard sized USB port on my iPad, when I connect my photo dock device. But it's pretty useless except for its stated purpose: to connect to a camera and suck in photos.
However, apart from cost and space concerns, I don't see a real objection to having an SD card reader inbuilt. The addon reader kit is a kludge. It's an ergonomic nightmare and its easy to lose. Very unapple like.