I'd like to play Tekkit. I'd also like to play it on 1.5.x.
Is this possible? Does Tekkit support 1.5.x?
No, at least, not currently:
The Technic Platform also offers Tekkit Lite which appears to have a similar mod list, but without the Bukkit portion, and the Feed The Beast packs also include similar mods, but the Tekkit that was in 1.2.5 is still in 1.2.5 until ForgeBukkit can be replaced.