Marriott did not waste a second from when the merger was approved to when their loyalty programs were able to be linked. It's fairly impressive given how long merged accounts on American Airlines and US Airways took. Don't freak out though, Starwood's SPG program is still here to stay for a while, in the meantime you are able to take advantage of some great reciprocal perks.
The ability to link your SPG and Marriott Rewards accounts is now live. I went through the process this morning and it was quite easy. I will give you advice, if you plan to link your accounts, I suggest doing this from your computer. My attempts to link from my phone caused some errors and I had to resort to my computer.
Above you can see screenshots from the SPG website. If you are a Marriott Rewards member and prefer to start from their website, you can do that as well. My preferred program is the SPG program, so I started on the website I'm most familiar with.
Using the link on the SPG home page, you'll be asked to log into your SPG account. After that you'll be asked to log into your Marriott account. A few prompts later you're all done. It is an incredibly simple process and very quick. In seconds my accounts were not only linked, but Marriott had already granted me Gold status with the Marriott program, since I have Gold status with SPG.
Once your accounts are linked, you can also transfer points between accounts at any time. The transfer rate is 1 SPG Point to 3 Marriott points. Transferring points will mean options on both sides of the equation.
SPG doesn't have a great partnership with United, one of the few that SPG doesn't have a great transfer rate for points. Marriott on the other hand does have a good relationship with United, meaning you can actually transfer SPG points to Marriott, then to United and get a better than 1:1 transfer rate from SPG to United. If you were to go directly from SPG to United, you'd get a terrible 2 SPG points to 1 United mile transfer rate.
There will be lots of opportunities to take advantage of now that your accounts can be linked. Go get yourself linked up while it's hot.