Ok, for those who don't mind a bit of work, you can do this already. Conveniently, the Brony pack has, indeed, updated to 1.5.1. Now, I'm assuming that no one here actually uses that pack, right? Ok, launch and install the pack, let it run once. Now quit out and go back through and click the edit mod pack button. Disable all the mods. ALL OF THEM, make sure you check all three tabs.. Close and relaunch, if you've done this step right, you will still get that silly little animation on launch, but you will end up in Vanilla 1.5.1. Quit out. Now, go out and grab the latest Forge, and again click the Edit Mod Pack button. Add the Minecraft Forge to the Jarmods section (for those wondering, for some odd reason the Brony pack uses liteloader, and not Forge). Close and relaunch. Now you have Vanilla Minecraft 1.5.1 and Forge.
A bit complicated, but from there on out you now have your own Forge version of Minecraft 1.5.1 to play with as you please. Just ignore any update warnings. Oh, and if you want to get rid of the clutter, go into the Brony folder and delete anything in the mod directory, just not the mod directory itself.