First, let's discuss the pros and cons to the SEO. If we are coming in after the design is completed there are more than likely (and by that I mean 99.9% likely) some structural and or major design issues that will need to be addressed.
Unless the web designer is skilled in SEO-friendly design we are going to have our work cut out for us just dealing with structural issues. On the positive side of this scenario, we're left to just work on the site with no designer taking it personally when we tell them we need to make changes (and that can lead to some issues).
Now let's look at the other scenario. Working with a designer has it's cons which is generally – we're going to have a designer taking it personally when we need changes made to the structure of a site or challenging us when we're arguing that while an all-Flash site is visually appealing, it leaves something to be desired from and SEO standpoint.