Human embryonic stem cells have been studied only recently, so their capabilities are, as of yet, unknown. In theory, the embryonic stem cells are able to form every cell type (which is what they do in the embryo). However, the conditions in culture might not be able to recreate the conditions that give rise to many tissues in the intact embryo. In addition to these unknowns, it is uncertain that the cultured stem cells will function the same as cells that have been developed within the embryo. For example, in a recent study, insulin-producing cells derived from murine embryonic stem cells failed to produce the insulin when transplanted into mice, but only formed tumors.1 In addition, it is uncertain that these lines will continue to proliferate indefinitely without undergoing genetic mutations that render them useless. According to the President's Council on Bioethics:
"It is not yet known whether any preparation of human ES cells (generally believed to be much longer-lived than adult stem cells) will continue to grow 'indefinitely,' without undergoing genetic changes." ²

References
S. Sipione et al. 2004. Insulin expressing cells from differentiated embryonic stem cells are not beta cells. Diabetologia 47: 499-508.
² "Recent Developments in Stem Cell Research." Monitoring Stem Cell Research. The President's Council on Bioethics, January 2004.