How should "et al." be used for parenthetical versus non-parenthetical citations using the APA citation style?
I think that the form


  Smith, Jones, and Roberts (2012) write that bla bla bla bla. Bla bla bla bla bla bla bla bla (Smith, Jones &amp; Roberts, 2012).


does not follow the recommended APA style.  That is, I think the correct way to write it is


  Smith, Jones, and Roberts (2012) write that bla bla bla bla. Bla bla bla bla bla bla bla bla (Smith et al., 2012).


where the first citation is the first citation "in text" and the second citation is the first citation of this work "in a parenthetical format."

Look at page 43 (figure 2.1) of the 6th edition of the APA manual.  Lines 2 and 3 of page 6 of the sample paper has the first citation of "Hahn, Carlson, Singer, &amp; Gronlund, 2006" in parenthetical form


  (consistent with Hahn, Carlson, Singer, &amp; Gronlund, 2006; Mather &amp; Knight, 2006)


and on lines 17 and 18, a citation of the same work in text is given as Hahn, et al. (2006)


  Similarly, Hahn et al. (2006) also found no age differences [...]


Now, it is not certain that the latter citation is the first citation in text of this work (because some text in the sample paper is obscured).  But looking at other papers in journals that follow the APA format seems to confirm my answer.

For example, a paper in the Journal for Research in Mathematics Education (vol. 43, no. 2, p. 133) (which explicitly states that manuscripts submitted to it "should conform to the Publication Manual of the American Psychological Association (6th ed.)") has a first citation in text


  In their article in Mathematics Teacher, Cirillo, Drake, and Herbel-Eisenmann (2009) situated their work on curriculum vision as described below.


Then a block quotation from this work ends in the first citation in parenthetical form


  (Cirillo et al., 2009, p. 71)