On X11 there are two clipboards - one that holds regular ctrl-c/ctrl-v values and the other one just for selected text. You don't need to do anything else than select the text to have it copied to the latter clipboard, you can select the text in any application using either mouse or keyboard and paste it later on by using mouse middle click (by default).
The Qt Creator 2.2 has a bug that affects this clipboard - only the text selected with mouse is copied, whereas keyboard-made selection is not.

Steps to reproduce:
1. open qt creator
2. open a text file
3. select some text using shift+arrows
4. press MMB somewhere else in the file to paste

Result:

• previous content of the clipboard is pasted

Expected result:

• text selected in step 3 should be pasted

Should work like this:
1. open qt creator
2. open a text file
3. select some text with mouse by dragging the cursor with LMB
4. press MMB somewhere else in the file to paste

Result:

• text selected in step 3 is pasted