cross processing refers to film being processed in chemistry for which it was not designed such as color negative film (C41 process) being developed in slide chemistry (E6 process) and v.v. - most labs will not do this so you may have to look around for a custom/specialist lab - try consumer/services forum
Only labs using either dark-room or manual machine setups will still do cross-processing... what happens is if you x-process CN film in E6 process the colors bleed and the slide chemistry is useless for other processing.
Fee Fee at hong lim will still do it if you ask for it, but they'll charge you a $6 processing fee.
Read somewhere that you should push your film and metering for 1/4 of the film ASA but develop at the rated ASA to get a better effect (ie. set camera and metering at 50 ASA if your film is rated at 200 ASA but develop for 200 ASA), but haven't personally tried.