It's definitely not a move up to the UFC middleweight division according to Johnson's manager Ken Pavia.  He spoke with MMAJunkie after Johnson destroyed Yoshiyuki Yoshida at UFC 104 and declared it as an isolated incident.  Johnson gave further explanation:

"I had an injury," said Johnson, who now joins the likes of Jon Fitch, Thiago Alves and Mike Swick as the division's top contenders. "It really held me back a bit from doing what I needed to do (to cut weight), and I got out of shape. But that's nobody's fault but my own."

It was a knee injury if I recall correctly, but as he said it's nobody's fault but his own.  I had thoughts of Johnson moving up to the middleweight division after seeing him dwarf Yoshiyuki Yoshida and missing weight by 5 pounds(he could have weighed in at 171).  However, after re-thinking it, IF he can consistently make weight at 170 then he would add huge intrigue to that division.  He has the size, KO power, and enough wrestling ability to potentially give the top guys problems.  However, he's only 25 years of age, and I can't imagine him staying at welterweight too long as his body naturally matures.  So he'll eventually have to move up to middleweight in my opinion.  With the KO of Yoshida, what's next for Johnson?  Should it be someone along the lines of Josh Koscheck?