After the reading of the haftarah, a series of b'rachot is said. The final one, on Shabbat ends, "m'kadesh hashabbat." On the 3 r'galim, "m'kadesh yisra'el v'haz'manim" (and on Shabbat fo the holidays, add in "hashabbat"). On Rosh Hashana and Yom Kippur, there is a change to address the specific day, as well.
But on Shabbat which is Rosh Chodesh, my siddur (and the one used by the guy who read the Haftarah in my shul, apparently) has no text incorporating "Rosh Chodesh" even though the selection which we read was special for Shabbat-Rosh Chodesh.
In Musaf, we end the middle section with the closing "m'kadesh hashabbat v'yisra'el v'Roshei Chodashim" so the text can exist and be used on the day. Why isn't it employed after the haftarah?