I 've been on Lamictal a little over 2 years, the "dangerous rash" called Stevens Johnson syndrome (can't remember it) occurs as soon as a person starts it and should be reported or actually a person would get sick right then and there, it also is usually fatal.
The rashing you are getting now, may be from the med. combined with other things, or has your dose been titrated up recently? Increasing dosage should be done gradualy (sp?) to help prevent onset of rash.
I have found too, that certain makeups, lotions I have used have started to give me mild rashing, one case I can pinpoint, my makeup now has an ingrediant it didn't have before, a sunscreen component, and a new astringent I started using seems to cause a mild rash ocassionaly.
I have always had terrible skin so it would be hard for me to know for sure if it is the med. But my thought would be if it was going to happen, it shouldn't just cause rashing 2 yrs later.
Maybe perspiration, age, topical products are some possible culprits?
I get terrible dermatitis on my hands just from different soas and cleaning agents, for years, I have an excellent prescription gel called Diprolene by Schering-Plough, it's great, I even put it on a rash patch I had on my jawline and forehead, like magic it brought relief.
Sorry to digress, but check it out with your pdoc. it may be something other than the Lamictal, or something not agreeing with the med. Lamictal has been working so well for me, minor rashes I will deal with than to stop the med.
If there wasn't anything that could be done then I may consider a change, but for now I'll us my topical gels/ointments.
I wish you luck with this, let us know how you make out.

DE