The following is a group of occurances between the pair that could be seen as possible evidence to support the coupling. Now since there is not any actual proof that they are a couple, you can look at this any way that you want. They are all instances from the manga because, quite frankly, there are no hints of romance in the anime.
It all started with the famous "one picture," of course, pairing each one of the soldiers to their general counterpart.
Since Rei rarely lets anyone get close to her, especially a man, the cuddly position they're in could possibly be viewed as evidence.
To view the "one picture" in its entirety, click here.
When Jadeite takes Bunny and Rei into his "alternate dimension" via mass transit in the 3rd installment of the manga, he approachs Rei, kneels before her, and says,
"How pretty. She's something special...this girl."
Since Jadeite is a servant of the darkness, it would be unusual for him to feel anything even similar to love, including such attraction.
Rei then transforms into Sailor Mars and kills him. ()^^
Super special thanks to my friend Usagi for scanning the above images for me.
Rei falls in love with a man named Kaidou, her father's assistant, in the side-story, Casablancas, which was featured in the manga #11. There is a scene where Rei sees Kaidou's face blur into Jadeite's. Rei screams something about not liking to dwell on the past.
For more info about Casablancas and Kaidou, check out these sites:
UsagiandMamoru.com (complete translation)
Fire Senshi's Realm
Silver Memoirs
