Tyco Electronics makes a NM cable interconnection device that is designed to be concealed in the wall. Care needs to taken that there is no violation of the NEC.

For more information on these devices see the following sections of the 2005 NEC.
545.13, 550.15(K), 551.47(O), 552.48(N) and 690.32. These are the only places in the 2005 NEC that permits the interconnection device. For more information see UL file number E57250.
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Mike