As usual, a lot of misinformation here. Unless Honda applied for and obtained a California Air Resources Board (CARB) certification for hiway use (on road) the bike cannot be legally plated in the state of California. PERIOD. Notice I said 'legally'.
The exceptions come from people finding someone that will error in issuing a plate for one that is not legally plate-able. Sometimes when moving into this state and transferring the title from another state to CA, they just process them through. The VIN Verification form (required on out of state transfer) has a box to check for the existence of a CA emissions sticker, the verifier checks it, signs it, enters their badge number and phone number. Even legally plate-able bikes have had trouble getting a VIN verification if that sticker is missing. A 50-state bike will state such and there is usually wording for CA state as well.
The CARB website lists all of the applications and certifications for on-hiway approved motorcycles - check it out if you don't believe me. KTM EXC's starting in 2007 has them for the 450/525 series. I had a 250 and the certifcation for greensticker is easy to find. There isn't one for on hiway. It costs the manufacturer/importer aproximately $35K PER MODEL to apply for these certifications. A good example of a screw up is BMW with their G450X - they neglected to apply for and get their CARB certification when the bike was introduced so they couldn't sell it in the hottest bike market in the world. Major screw up. The next year they got it, but the bike's reputation had already caught up with it (it's a POS) and by then no one would buy them unless they were deeply discounted. So they just chucked it in and gave the whole kit and kaboodle to husky to build the TE449/511 series. The G450X became an instant white elephant. The only honda dualsports that are legal are the 'L' models and the new CRF250.
I went so far as to create an emissions sticker for my 250, using the same format as it's bigger brothers. You couldn't tell it was a forgery, I'm pretty good with Illustrator. I know it would have passed inspection. And I got a lighting verification affidavit, and could have easily got a plate for it. The problem is the forged emmission sticker, if caught, is a felony. It wasn't worth going to jail for a license plate for a dirtbike. Not at my age, not with my dependents and responsibilities.
So the comments that 'there are lots of them that get thru' comes from the fact that lots of guys have gotten them, but there are many more that didn't and couldn't. They have it just as bad in WA state and they even went back and pulled thousands of them dealers put on when they weren't supposed to. Imagine how all those guys feel. WA is just as much of a nanny state as CA. Go read in their regionals about their plating headaches.
So yes, you can try to plate a 450X and you might very well get one. But you might visit a dozen DMV offices and get turned down as well. There used to be a few dealers that would get anything plated with your existing CA Title for a fee, usually $500, but all those guys have been busted already. Hence the advice to just get a KTM or a Husky and be done with it. Or a FE570, or seems to me a model or two of the Beta's can be plated. Or buy one with a CA plate already on it - that's how I got my WR250F with a plate. The original owner moved to Louisiana, plated it there, and when he brought it back to CA some DVM idiot transferred it. DMV has two files on that bike, one for offroad (it was a red-sticker in '04) and one for the hiway plate. That license plate is worth about a grand.