So, I'm thinking about using a mini-arc to ease the PCs in to the Planes. Give them some exposure to things in the planes, hand out some free planar knowledge points for them to pick some interests, and generate some far-flung contacts. May help them choose eventual feats and Prestige Classes, or even whether they really want to go to the planes.

Basic Plot Summary:

Drop in teaser encounters

While following main Party driven plot; PCs get summoned several times to various locales in the planes by spellcasters. Range of situations- combat, servants/errand boys, Scholar/Diviner/Researcher, etc. [Maybe give 50% Combat XP for PCs that die while summoned.]

Meat of the Story

PCs Discover that a "Summoner's Broker" acquired their true names/lock of hair/whatever (Focus-for flavor and storyline not the optional rules set) and has begun selling them to spellcasters in and around Sigil. Spellcasters buy (a Focus) to ensure certain abilities/classes.[Let conjurer tell PCs if they have no means to discover the truth]

PCs head to the Planes under their own power(borrowed, bought, stolen, or earned) to track down and deal with the Broker. Lead them around on a short chase/tour over the course of several encounters before finding Broker. [I'm leaning towards Roleplay over Combat for Final Encounter here. Let the Broker tell them about primes, planars, petitioners, etc. Whatever else they run into along the way can kick their asses before hand.]

Afterparty
Door remains open for a few more 'summoning' adventures or a means to transition to a planar campaign.