(Kokusyo Sayuri). Lucky Star #6 ED. The actual song starts at 1:19.
This is a popular wedding song so it's one of those oldies people still sing in karaoke's. This is her only hit single. In fact the first time I really paid attention to this song wasn't because of Lucky Star, but Japan's #1 TV Show, London Hearts
(don't know how many JP-based Otaku would watch LonHatsu though).
The now 40 years old ex-idoru Sayuri-chan has now become a divorced actress-comedian famous for her grumpiness. She's a LonHatsu regular who talk shit about everything but mostly about men. She's actually a classic Tsundere now - sharp tongue but a soft heart. A while back London Hearts gave her a fabulous 40th birthday gift: a surprise concert where she gets to perform "Valentine Kiss" one more time in front of hundreds of fans in her original outfit (they even custom made one for her "current" size). She wasn't told about it until minutes before she show start though. It was pretty awesome to see her looking like a 17-years old again and everyone including us got a bit emotional. I'll tell you Japanese TV has some really good stuff, even outside of Anime.
EDIT: I just found that she has some anime roots too - according to her JP wikipedia page she guest-stared (as Seiyuu I guess) on 2 episodes of City Hunter anime. I found a link to her London Hearts Concert on Youtube but it was deleted. Damn you Asahi Terebi!
EDIT2: I also realized that the LonHatsu concert happened during Jan of '07, right when they're producing the anime. Could this be the reason why they used this relatively obsure song? I heard many of the animators in KyoAni are women, and women is the biggest audience for London Hearts. Why knows, maybe Sayuri fan at KyoAni asked to put this song in.