I am of the opinion that gnomes are the best spellcasters for pure spellcasting, and are great for sorcerers.
First, for the reasons already pointed out: Small size is great for spellcasters. They typically don't need to carry much or do much damage with weapons. However, they're +1 AC, +1 to hit, and +4 Hide--all useful advatages for any spellcaster.
-2 Strength isn't much of a drawback, for the same reasons it's not a problem on a Small character. Halflings get this penalty, too, so if you're small, it doesn't really matter.
Gnomes don't get the +2 Dex that halflings do--so they don't get an additional +1 AC and +1 to hit (rays and touch attacks with Weapon Finesse, I'm thinking). Then again, there are plenty of spells to choose from that don't require pluses to hit: Magic Missile, Sleep, Color Spray at low levels, Fireball and Lightning Bolt at higher levels, and so forth. And that's just the offensive magic. Gnomes also get something that's arguably much better: +2 Con. That's +1 HP per level, which means you'll have at least 25% more hit points than the average sorcerer. If you average 2.5 HP per level (average roll of 1d4) then you're gaining 40% more hit points than other sorcerers. I once played a gnome spellcaster with a 16 Con--The average roll on 1d4+3 is 5.5, which is equivalent to 1d10 (okay, 1d8 and 12 Con...)! I was just as tough as everyone else but the tanks--and tougher than some. +2 Con also gets you +1 to Fortitude saves, and +1 to Concentration checks.
Furthermore, the Illusion save bonuses are nice. It's great for shadow magic and Color Spray, of course, but depending on how creative you're feeling, regular illusions can be very game dominating. Bonuses to Alchemy are excellent.
The piece that seals it for me, though, are the extra spells. A gnome sorcerer has their own level-0 spells, and Dancing Lights, Ghost Sound, Speak with Animals, and above all, Prestidigitation. Once again, creativity can make all these spells very useful in lots of situations. Intimidating and bluffing become easy when you're tossing lights, images, and sounds in all sorts of directions.