ASM Software was able to build and run the 8051 emulator emu51 (http://emu51.sourceforge.net/) under GNU/Linux.
Download the source distribution, make sure you have the Allegro library installed, and now compile emu51 with this Makefile.
The emulator seems to work but keep in mind it is very early alpha and the interface not that easy.
If you use SDCC, make sure to issue the packihx command first. emu51 has problems with reading the .ihx output of SDCC properly.
Once a port to the SDL library was planned but the project was abandoned due to lack of time.