I'd go for uCLinux . It's been ported for ARM, it has strong user base and a decent portfolio of evaluation boars exists. There are couple of other Linux/ARM projects around and you'll see.
I'd here the port for uCLinux for '51s but google did not show???????
plz where is it????????
does keil support 256 banks now? it seems only 64 for Lx51
does keil support 256 banks now? I do not know, but you would need at least that many to support Linux.
Erik