Looks like a large wildfire in the Cajon Pass area closed I-15 for a stretch today, and the SB lanes are still closed.
LA Times: http://latimesblogs.latimes.com/lanow/2011/09/northbound-lanes-of-interstate-15-reopened-brush-fire-along-cajon-pass-continues-to-burn-.html
This is not good on the Friday of a long weekend as tons of people make the trek to Las Vegas. There aren't really any alternatives: I-5 over the Grapevine to CA-138 to CA-18, or I-10 east to CA-62 and CA-247 through Yucca Valley. I-10 to US 95 is probably not out of the question either....
14 to 58 would work as well.
Yeah, I'd say the best alternative is to take the 14 to the 58 to the 15. Almost all of that is now freeway, and if coming from the coast, it's not really that much longer, anyway.