Ooh, I do hope that lots of Ancestry customers will sign up, so I can compare more people with my 23andMe matches who aren't on Gedmatch.
On 23andMe, "reports" are completely separate from the DNA Relatives "tool" and the Share and Compare "tool" for DNA comparisons. I read the 23andMe message as indicating that people who upload data will be able to use DNA Relatives and Share and Compare (the online tools for exploring relationships), and they get 4 reports as a bonus.
That page that Jamie found is where they explain that 23andMe customers aren't required to participate in the DNA Relatives tool, but they will get a one-time DNA Relatives report even if they choose not to use the tool.