I could be totally, wrong, but there's a TON of stuff supporting this idea.
Let's not forget MGS2 where we THOUGHT we would play as Snake, then when Raiden shows up, he goes under the name of "Snake" for a little while before getting his name changed to "Raiden". So "Raiden is Back" only confirms the codename. Now let's move on to the evidence...
EXHIBIT 1) SUNNY Days Are Here Again!
On the "NEXT" website, the "Raiden" thunder (god) storm is over. And it's SUNNY again. And at the end of MGS4, Sunny herself said "Sort of like the sun...it's RISING again!"
EXHIBIT 2) Flowers Blooman
When the website's page shifts to the wall, we see a picture of "Raiden" and a FLOWER in the upper right corner. Who wore a flower in that same corner? Why it was SUNNY in MGS4!
EXHIBIT 3) Dose Eyes!!
Those eyes look rather similar, don't they? Granted Sunny's eyes are a little lighter, they're the same pigment. And when you look at "Jack the Ripper" Raiden's eyes, you see that his are CLEARLY blue and not orange/reddish.
EXHIBIT 4) Dat Face!!
That face looks pretty feminine doesn't it? Even for "Jack the Ripper" Raiden, This is a combination shoop of the new "Raiden" face and Laughing Octopus' face from MGS4.
EXHIBIT 5) Dem Accessories!!
Check out some high res shots that flashed in the new trailer for MGSR..
Hmm? "Use designated tool to remove fastener?" What could you be fastening? Maybe dem grown-up titays?
That's a stretch, but you never know. However, there's also this other one...
Those sure are some FABULOUS nails for Jack the Ripper Raiden, aren't they? HMMMM...
Oh hay, Raiden had those kinds of nails in MGS4 LOL mah bad n/m!
EXHIBIT 6) Dat Title!!
Oh hey, the game's name is "Metal Gear Solid RISING." Like.. a RISING SUN ("Sort of like the sun...it's RISING again!"). Or THE RISE OF SUNNY (to epic ninja status?). HMMMMMMMMMMMMMMMMMM!
So that's all the evidence I have so far. Sure, this is all speculation, but you have to admit there's a strong possibility this is true. It would actually make sense for Sunny to take on the "Raiden" codename since Raiden was who rescued her from the Patriots.
And let's not forget. Kojima LOVES to fuck with our heads.