The most widely held practice regarding heating up liquids on Shabbat is that it is prohibited, though there are differences of practice between Ashkenazim and Sephardim regarding sauces and such (Sephardim are generally more permissive).
There are Yemenite Jews who practice strictly according to the Rambam's rulings and therefore they will lechatchila, a priori, (i.e. it is 100% permissible) remove cold soup from a refrigerator and place it on a hot plate for heating purposes. For an interesting shiur on the subject from Rav Melamed, rabbi of the Har Bracha yeshiva, see: http://www.yeshiva.org.il/midrash/shiur.asp?id=642
My question is the following: I was told by an old friend of Atara Twersky, the daughter of Rav Soloveitchik, that she also used to heat up soups on Shabbat. I don't know if this means that she was following the practice of her father, or her husband, the Talner Rebbe, Rabbi Isador Yitzchak Twersky.
Does anyone have any knowledge of Rav Soloveitchik's halachic rulings in this area? Are there others who follow this practice?