After reading your post, I realized that the same thing has been happening to me! My little boy is nine months, and I nurse him morning and night and pump two or three times a day at work. He has between 12-18 ozs. milk while I'm gone, plus rice cereal, and three fruit/vegetable containers (Gerber). My pumped milk usually doesn't break even with what he drinks while I'm gone, so we supplement with formula. (I used to get about 20-24 ozs. pumped a day; now I'm lucky if I get 10-12 ozs.. My job as a teacher is pretty high-stress and insanely busy, which is horrible for milk production, I'm sure.)
For me it's a really emotional and stressful thing to make enough milk for him (probably to assuage my guilt at not being home with him!)--and I feel like I'm slowly losing my supply, and I hate that feeling. For me having a slow letdown feels like it's more evidence that I'm not making enough milk for him. He also nurses for less time lately, again making me think there's not enough there for him.
I guess I'm just writing to commiserate. Maybe I'm wrong about losing milk? Maybe the other mothers who wrote to say that a slower letdown doesn't mean anything are right--I hope so!!!