Here's a list from 2channel of "common-knowledge seiyuu couples." Almost completely without evidence, but fun to contemplate. Just to make this even more like a tabloid rag, I've included photos.
Kamiya Hiroshi (34) and Saitou Chiwa (28). The story is that after a difficult divorce (from a mangaka), and then a devastating motorcycle accident, Kamiya found Chiwa was his true friend. Some of the detailed indications on this 2channel page make at least a bit of sense: he gave his girl friend a watch and Chiwa likes watches, etc. The two have worked together quite a bit, particularly as Kouhei and Hazuki in Tsukuyomi Moonphase and as Araragi and Senjougahara in Bakemonogatari.
Nakamura Yuuichi (29) and Inoue Marina (25). She is a capable and serious ojou-sama. His motto is "never be late" -- but he is apparently also highly sociable and the last to leave a drinking party. Looking at this site, for instance,this seems to be a rumor with some reasonable circumstantial evidence behind it.
Taniyama Kishou (34) and Tomatsu Haruka (20). Well-known rocker Taniyama was previously (and probably spuriously) linked with Hirano Aya. At least the stories are consistent: the old satyr and the slim and genki young entertainer.
Hino Satoshi (31) and Nakajima Saki (31). Shana's foil and Saki from Hayate.
Ono Daisuke (31) and Tamura Yukari (33). Two mature, capable, and successful seiyuu.
Terashima Takuma (26) and Itou Kanae (23). Teppei from Princess Lover and Amu from Shugo Chara.
Okamoto Nobuhiko (23) and Hikasa Youko (24). Layfon from Chrome-shelled Regios and Mio from K-On.
Irino Miyu (21) and Sanpei Yuuko (23). I think YuuYuu would wear the pants in this family. This match-up makes an interesting contradiction with the supposed yuri pairing between Sanpei and her old friend Koshimizu Ami. In fact, they have been showing up in each others' blogs somewhat less often in recent months.
Suzuki Tatsuhisa (26) and Katou Emiri (26). Yuuji and Hideyoshi in Baka to Test. Or Sugata in Sora no Otoshimono and Kagami in Lucky Star.
Suzumura Kenichi (35) and Sakamoto Maaya (29). Two of the beautiful people.
Washizaki Takeshi (36) and Iguchi Yuka (21). The smart radio/TV host and the young veteran who plays moe roles like Index.
Yoshino Hiroyuki (35) and Ueda Kana (29). Allelujah Haptism in Gundam00 and Saki in Saki. Kana-chan is a lot more mature as a person than are the moe characters she tends to play.
Sakurai Takahiro (35) and Mizuki Nana (30). If this is true, it's huge. The wildly popular Nana-chan and a star seiyuu who remains under a bit of cloud for plagiarizing a live drama he helped produce a couple of years ago.
Tachibana Shinnosuke (31) and Sakuno Airi (25). Not so well known, but they both work for big agency Production Baobab. She plays mainly unnamed roles. He was Minato in Sekirei.
Nabatame Hitomi (33) and Itou Shizuka (29). At least there's one yuri couple that may actually be a couple. Two strong women who are top professional voices.
Morikubo Shoutarou (35) and Nakahara Mai (28). Shou-chan is one of the foremost boys-love seiyuus. Mai-sama -- who just seems to get better and better at what she does -- was formerly linked with Shimizu Ai in a (fake) yuri relationship.
Just to repeat: I have no knowledge that any of these relationships actually exists. This post is just for fun. My only basis is an otaku's post on 2channel.