I wrote this syntax highlighting file for Vim because I had an Intel 8051 course in 2005, and wanted to do my coding in Vim. It's totally optional to have syntax highlighting, but it does give you many advantages. This .syn file will make sure recognised mnemonics and registers are given a nice color. Also values that are entered are only highlighted if they are valid. Note that the colors depend on the color theme you choose in Vim, no fixed colors are used. You can easily add extra registers specific to your 8052 to the .syn file, the file should be self explanatory.
You can find the syntax file and a (very basic) ftdetect file in a zip archive on my download page .