This is a really great question as it has lots of implications for our futures and that of our offspring. There are so many variables that impact when and how we tell our partners such as the length of the relationship, the depth of the relationship, our communication skills, our awareness, understanding of and attitude towards thalassemia and its implications, our personalities etc. In some cultures and societies there is a lot of stigma associated with thalassemia and people prefer not to talk about it.
If the relationship is brand new it is not always easy to go on a date and say "Hi my name is X and I am a thal". Mind you it can be a very educational opportunity for a prospective partner.
In my case, I only recently confirmed via DNA analysis that I have an alpha thalassemia point mutation (thal minor). Long story! However I always suspected I was thalassemic and assumed I was even though the full evidence was not there at the time. It was both my gut feel as well as my risk management strategy! This is in fact a problem because lots of people could be carriers and not know it - especially silent carriers of alpha thal.
My attitude towards my "thalassemic status" was to be open about as I felt that the person I was meant to be with would accept me for all of me, including my thalassemic genes. So when I started dating my now husband many years ago, I told him very early on in our relationship. He was great, very proactive, went and got tested. In hindsight he should have done a DNA test to gauge whether he was a silent carrier but DNA sequencing tests were not readily available 16 yrs ago here in Australia. Based on his FBC and other blood work he was not thal however but prior to being tested he said either way we would work through any issues that would arise; this in fact helped cement my relationship with him and set the foundation for working through other trials and tribulations we have been through since then.
Since then we have had a couple of miscarriages and a healthy girl, who is our joy. I had a hard time coping with the miscarriages so it was interesting to read the comments on this site about thals and miscarriages. I may need to explore this further to get complete closure over these losses as I had no identifiable reason for why I lost two babies in my first trimester.
Back to your question, the way I see it is that amongst the many conditions that people may have, thal is one of the few that is now more easily identified using genetic testing; even in the carrier state, despite all the mutations that are being discovered all the time. I think it would be fair to say that many other conditions have yet to be genetically mapped and properly understood. In fact we can all be carrying latent genetic conditions that we are oblivious to; that may only express themselves under certain conditions.
My motto is that "we are more than the sum of our genes" and transparency and awareness about topics such as thalassemia are very important public health issues that should be discussed. Furthermore apart from prospective partners whom else should we discuss this with? Today I told my child's physical education teacher about my child being a likely thalassemic carrier and the impact this may have on her at sport. She was struggling when doign her cross country at school and the teacher was pushing her which created unnecessary stress for us both. So we had a great discussion about Pete Sampras; it also helped that my child had a book about him which discussed his thalassemia b minor and his strategies to overcome this as a sportsman. Now the sports staff at the school may think twice before pushing kids too far with strenuous sports and customise a physical education program more suited to their needs and genes.
All the best.