Quite honestly, why would you want to, as a sorcerer, waste a feat that could be spent on something amazing like Sudden Maximize in order to get Diplomacy as a class skill? And give up a caster level??? Horrible idea for almost any caster in D&D. Unless you're playing a game with virtually no combat where negotiations and roleplay is the heavy focus (in which case why are you using D&D?) then a sorcerer with cross-class ranks in Diplomacy is going to do fine.
Let's make some simple assumptions here. You're a sorcerer with a minimum 15 Charisma. As you go up in levels, you naturally put all your stat boosts into Charisma. By 20th-level you'll have a 20. Let's also assume by then you have a +6 Charisma item; not a big leap to make for 3e. You also take wish as one of your 9th-level spells and wish yourself to have +2 Charisma (or buy/loot a +2 tome of leadership and influence). So you're at 28, which is a +9 bonus by itself. Now let's assume you have a 10 Intelligence; sorcerers have better things to do than think. That's enough for 1/2 cross-class rank in Diplomacy, 1 rank in Concentration with 1 skill point to spare every level. Now at 1st and 2nd-level you keep Bluff maxed out as well so by 2nd-level you get a +2 synergy bonus to Diplomacy. After that, for the next 10 levels you put 1/2 cross class rank into Sense Motive. By level 12 you get another +2 synergy bonus. For eight more levels, you put 1/2 cross class rank into Knowledge (nobility & royalty), but because you want to reach that last +2 synergy bonus before you hit the magical level 20 you sacrifice 2 ranks of Concentration along the way. By 20th-level you have +6 from synergy. You're a sorcerer, so naturally along the way you'll pick up the greater heroism spell, which grants +4 to all skill checks. Handy. As the icing on the cake, you pick up a circlet of persuasion somewhere along the way, again, not hard to do before getting to 20th. So where does that put us?
+9 Charisma modifier
+11.5 Diplomacy ranks
+6 synergy
+4 greater heroism
+3 circlet of persuasion
Total +33 Diplomacy - all done using CORE RULES ALONE
That's enough to make a hostile creature indifferent as a full-round action taking a -10 penalty so long as you don't roll a 1. Do you honestly need 12 extra ranks of Diplomacy just to make the creature friendly? If you wanted to do that you would have cast charm monster, because you know, you're a sorcerer.
This is so totally not worth wasting a feat or losing a caster level over. Some things may be worth it, but this is not.