In my opinion, if it is not too annoying, it is acceptable. As I wrote in my comment to the question, this should not make me think about many things at the moment, so "join the discussion" message can force me to read the text without caution because I'm thinking what a great discussion I am going to join in a minute.
This concerns all the web/application elements. If I'm reading/watching picture or video, I don't want to be disturbed, especially by "do you remember you can add me to your friends?" or something like that.
In my comment I gave a good example of Stack Exchange sites About pages. The next part is shown at the moment when I finish reading the previous. This is acceptable and nice. If it disturbs me, I will quickly learn to be "banner-blind".
There is much reason to delay GUI elements to make sure the user does not click/activate mistakenly. For example (again SE) the review page -- you can't click too quickly "I'm done" button. You should not be able to accept at first "Install new software?" question (like eg. Firefox does). In old Windows installers you could accept the licence agreement only if you've read all the terms (which was leading to click in the text box and press PageDown key until end, but I think the idea was good).
Sometimes it is done so, that eg. a web mail provider wants to show new options in his application. So he puts dialog boxes in different parts of the page and shows eg. "Using this button you can quickly mark the message important". I click "ok" and the next pop-up shows. I must move my mouse to the other edge of the screen and click it, I do this without reading, because I'm strongly thinking what an e-mail I want to write and I will forget it soon.
So in my opinion it is acceptable, but only if this will not alter user's attention from more important things.