I just had this same problem and found a solution!! Whoop Whoop! :) Hope this helps you.
Really stupid actually!!
I am using the twenty eleven theme with buddypress...so I used this plugin http://wordpress.org/extend/plugins/bp-template-pack/ which adds support for buddypress to your existing theme.
After activating the bp-template-pack plugin...
Go to Appearance > BP Compatibility
If you disable CSS and refresh your "profile activity - what's new" page, you will see the button appear!! Use inspect element on the submit button and you will see a div structure like this:
<textarea name="whats-new" id="whats-new" cols="50" rows="10"></textarea>
<input type="submit" name="aw-whats-new-submit" id="aw-whats-new-submit" value="Post Update" />
[Please post code or markup snippets between backticks or use the code button.]
When you activate the css again, it seems as though the button disappears but if you look at the source code it is actually there it's just that the buttons parent div #whats-new-options has a height of 0!!
height: 0px; /*CHANGE THIS HEIGHT TO 40px or whatever you want*/
That fixed it for me! Hope it works for you guys! :)