Set to appear on episode 19 of Arrow is Caitlin Snow, played by Danielle Panabaker, and Cisco Ramon, played by Carlos Valdez, known in the DC comics as Killer Frost and Vibe.
Both actors have been announced for The Flash TV series as well, though not specifically as their DC comics counterparts.
Interestingly enough, episode 20 of Arrow was said to feature Grant Gustin before going on to star as The Flash in his own series. Reportedly, when the CW greenlit the pilot for The Flash, episode 20 was changed; so perhaps we'll still get references to Central City with Snow and Ramon.
Here are the characters the call sheet reveals for Arrow episode 19 (via Spoiler TV):
Stephen Amell - Oliver Queen/Arrow
Simon Burnett - Oliver Stunt Dbl.
Katie Cassidy - Laurel Lance
David Ramsey - John Diggle
Emily Bett Rickards - Felicity Smoak
James Michealopoulos - Deathstroke Stunt Dbl.
Atlin Mitchell - Sara Lance/Canary (probably a double)
Danielle Panabaker - Caitlin Snow
Carlos Valdez - Cisco Ramon
JJ Makaro - Stunt Coordinator
James Bamford - Fight Coordinator
Hugo Steele - Security Guard #2
Colton Haynes - Roy Harper
Manu Bennett - Slade Wilson
Summer Glau - Isabel Rochev
The CW is currently filming The Flash in Vancouver, with Arrow airing Wednesdays at 8pm ET on the CW.