The Mariners' rotation, like the rest of the team, has been constantly evolving throughout 2011. The fact that three of the opening day (or opening week) starters remain in the starting five suggests false consistency.
Felix Hernandez, Jason Vargas and Michael Pineda have been consistent, rarely missing starts, but the other two spots have been thoroughly cycled.
It's not abnormal for the fifth and sometimes fourth spot to change up as the year progresses, but it is arrhythmic to trade away two of your starters at the trade deadline.
In the Mariners' circumstance, it was definitely the right decision, but since then, no pitcher has effectively stepped up and solidified a spot with eyes on a spot in next year's rotation.
We can be nearly certain that Felix, Pineda and Vargas will retain their posts, so the best way to look at what next year's rotation might look like is to rank the top five candidates for the four and five spots.