I see this in MonoDevelop (via GTK# on Mac OS X):
On OS X, shortcuts in textboxes are with in linux style instead of in mac
style. 
For example, you should use 'home' to go the beginning of the line, instead of
(cmd+Left).
Another example, you should use 'ctrl+A' instead of 'cmd+A' to select all text
in the text box.

Steps to reproduce the problem:
1. Type in any textbox
2. Use mac shortcut 'CMD+A' to select the text that you just typed

Actual Results:
The text was not selected

Expected Results:
The text should be selected

I believe that the best practice to fix that is to wrap the Cocoa's native textbox.

Other information:
See "Working with Text" section here: http://www.danrodney.com/mac/