Found this on the web:
There are two opinions in the Gemara (Yevamos 39b) about whether or not we should perform Yibum. The Rabanan hold that we should and Aba Shaul holds that we should not. The Rambam explains in a Teshuvah (Yerushalayim, 5694, #170) that according to the Rabanan the Isur of Eishes Ach b’Makom Mitzvas Yibum is completely nullified (Hutrah), whereas Aba Shaul holds that it is only removed if the Yibum is performed l’Shem Mitzvas Yibum (Dechuyah). According to the Rabanan even if one does not perform Yibum l’Shem Mitzvah there is no Isur.
The Rambam (Hilchos Yibum, 1:2) says the Halachah follows the opinion of the Rabanan. Rabeinu Tam holds the Halachah is like Aba Shaul. The Beis Yosef (Shulchan Aruch, EH 165:1) decides in favor of the Rambam and the Rema (op cit.) brings the opinion of Rabeinu Tam. According to this, the Sefardim who follow the Psak of the Beis Yosef, should be able to perform Yibum even today. The Ashkenazim who follow the Psak of the Rema do not practice Yibum at all.
A decree was issued by the Chief Rabbinate of Israel in 1950 and signed by both the Ashkenazi and Sephardi Chief Rabbis, that Yibum should not be practiced in Eretz Yisrael by anyone including the Sefardim, and that anyone that refuses to perform Chalitzah on his Shomeres Yavam is obligated to support her financially.
Rav Ovadia Yosef wrote a Teshuvah (Yabia Omer, E”H 6:14) stating that this decree has no Halachic basis and that the Psak of the Beis Yosef cannot be overturned (and he added that if they extract money from the Yavam to support the Shomeres Yavam, that it is Gezel). Nonetheless, the fact is that as a result of this decree, Yibum is illegal today in Israel, so it is not practiced by anyone here. It seems (from Teshuvos written by Rav Moshe Feinstein and other Poskim) that it has been performed in Chutz la’Aretz by Sefardim.