Update, May 2017: These requirements have now been replaced. See: Second Life mesh upload prerequisites revised.
Back at the start of June, we learned that the ability to upload Mesh to SL is to be gated. You can currently take a peek at how this works by going to your Beta (aditi) Grid status page.
Click on the right-pointing arrow next to Account to display your account options, then click on MESH UPLOAD STATUS (located between SCRIPTED AGENT STATUS and CHANGE PASSWORD, as circled below).

To qualify for Mesh uploads, you must provide payment information to Linden Lab, and complete a Mesh Intellectual Property Right tutorial / questionnaire, which comprises a series of multiple-choice questions, as shown below.

The questionnaire is not mind-bogglingly difficult – and it shouldn’t be, the idea is to make people aware of what will not be tolerated and can lead to their ability to upload meshes being revoked; not to try and catch them out before they even get started. As such, it is possible to go back a step should you get any question wrong and then take the question again.
Completing the questionnaire results in you being informed that you are now cleared to upload meshes, and provides a link to some additional information related to trademarks and copyright under US law. I was actually surprised that there are (currently, at least) no links to Linden Lab’s own policies around mesh and IP.

Important Notes!
- This is still currently *only* available on beta grid dashboard pages (https://secondlife.aditi.lindenlab.com/my/).
- You won’t currently see the option if you are using your main Grid dashboard page (at https://secondlife.com/my/).
- It is possible what completing the questionnaire at this point in time may only apply to the Beta grid – you might have to complete the questionnaire again once the link is available from main Grid dashboard pages.