Need Realtek's rtl8188cus (WiFi USB dongle) datasheet. Any advice?
I expected this to be a problem of an OEM-only part with everything under NDA and binary drivers provided.  You'd need to be planning a large-volume order (and have the company letterhead to prove it) to get anything that could help you write a driver for many components like this. 

So no, don't expect this to be as simple as Googling the part number and finding the datasheet in the top link.

I went to the Realtek site and looked for documentation, expecting to post a link to the "Volume orders only" page.  However, I was pleasantly surprised when I went to the WLAN NIC > IEEE 802.11b/g/n Single-Chip > Software page and found Linux drivers.  I had expected Windows binaries, but figured you're probably running an embedded Linux stack, and was about to post some information on configuring your real-time embedded Linux stack to accept the drivers designed for desktop Linux (Ubuntu 10.10, in this case).  However, I waited for a moment and let the download finish. On opening the file "92ce_se_de_linux_mac80211_0005.1230.2011.tar.gz", I was still more pleasantly surprised to find C source code!  And a readme!  And release notes!  And firmware binaries to flash to the chip! There are even comments in the code! And, the most surprising thing of all, there's even an email address to contact the maintainer, Larry Finger, who is also listed on linuxwireless.org/en/developers/maintainers.

The datasheet is probably still under NDA, but you don't need to write drivers. You have them already written for you.

Do note that the source is under the GNU General Public License, Version 2.  If you use this code in your project or make changes to it, you must release it and anything into which it is compiled (likely including the OS) to anyone who receives the source code.  That includes customers who purchase your device and receive the source code as firmware flashed into it.  That's a small price to pay when you consider that you have the driver source code available to you already.