Having myself been accused more than once of taking my SO for granted, I think that, while it certainly does happen, it is also a common misperception for something quite different.  You partially hit on it in your question, actually.  I call it comfort level.

For an analogy, consider the common patern of many other friendships.  When you first meet, you put on your best show, try to maintain some pride, and generally are a little self consious at all times.  After you've known someone for a long time, they've become a close friend, seen you snoozing on the couch a few times, seen you eating breakfast before the sleep lines are off your face, helped you through a few bad days, and etc., you're much more relaxed, don't generally feel such a need to impress your friend, and don't worry so much about planning meetings.  The difference isn't that you value the person less.  Actually, it's quite the opposite.  You two have gained eachother's trust (we're considereing good friends here), and you consider each other known quantities that can be counted on.

I think "partner" is a very good term because it exemplifies exactly how the relationship should become. Partners in a good team know each other well and don't need to pay as much attention to eachother as they do to everything else, because they know they can count on their partner. Consider, for instance, basketball or soccer teamates. I'm not much of a sports nut myself, but I know that good players are able to pass the ball and to work together keeping their eyes more on their opponents than each other because they know what their teamate will do, where they will move, and how they will react. This level only comes with experience, so it's not like that with new teams.  If this sounds like a very male analogy (using sports to explain life) consider that it may be because I'm explaining a commonly male mindset.

The most obvious difference I notice between being taken for granted and just being comfortably accepted is the value placed apon the person.  I've seen guys that obviously take their SO for granted and it shows because their actions say they don't value her by negative treatment, such as insults, intentional exclusion, mistreatment, and general apathy in her direction.  On the other hand, guys that are quite comfortable with their SO may treat her as "one of the guys," which, although it may seem like a downgrade in outward actions from when the relationship was new, is actually a status that many strive very hard to attain because they know it signifies acceptance and validation.

So I guess the question boils down to: does he actually treat you negatively or does he treat you more like a known quantity/good partner?  Since you said in another post that "he sure doesn't seem to think [you]'ve lost [your]... appeal" it sounds to me like he values you quite a bit and would feel hurt if you suggested to him that he is taking you for granted when he actually likes you so much that he is just more comfortable around you than ever before.

Of course, this is just MY personal view from having been the guy accused of taking an SO for granted before even though I really did appreciate her; not kowing your SO I can't speak for his intentions or the reasons behind his actions (or lack of actions).