Originally Posted by rschleicher

It seems to me (from memory), that the RD from the prior iteration of 105 (5600 series) has a stated maximum cog size of 27 teeth. I have 5600-series 105 on my bike, and when I got it I had the 12-25 cassette changed out to a 12-27. At that time, there was an 11-28 Ultegra cassette available, but not for 105. I seem to think that this was a limitation in the 105 RD. The new 105 (5700) will support an 11-28, in any case....

That said, even if the above is correct, you may still be able to make things work with some adjustments, since the size difference between 27 and 28 is pretty small. Barring that, perhaps a new RD will solve the issue (either Ultegra or the new 5700 105), if you really want a 28-tooth cog.