Not Terribly broken, but a tome dragon w/ loredrake would be quite good.
In general, I find it amusing that this is actually pretty balanced (because dragon class features tend to be so mediocre).
Best,
David
EDIT: The previous poster is also right about the steel dragon.
Looking over Tome Dragon, it might actually get ahead of things in the spellcasting department with the LA ignored.
Tiny Wyrmling Tome Dragon
3 HD
LA 5
Caster Level 3rd
11 SR
Arcane Sight
Precognition
Telepathy 100ft
-4 str, -2 con, +6 int, +4 wis, +2 cha
Alternate Form
Graceful Flight
Physical Frailty
Automatically knows all Sorcerer Conjuration and Divination Spells. Access to Knowledge Domain spells. (No access to the cleric list as far as I can see)
With LA ignored, by default the Tome Dragon is at 1:1 sorcerer casting progression. Throw in Loredrake, drop to d10s, and it is 2 levels ahead of standard sorcerer casting, 1 ahead of a wizard.
Moving up to Very Young Loredrake Tome Dragon, another +3HD for 6 HD total gains 5th level casting, and Free Metamagic +1 including no increase in metamagic time. That puts them right on par with an equal level wizard with some pretty sweet features and a stat array mod of -2 str, +6 int, +6 wis, +4 cha. Pretty much ready to PrC into something right away. No matter how you slice it, those dragon HD are better than sorcerer levels.
With the exception of Shadow Miracles, you can even compete with Shadowcraft Mages in the spontaneous department.
Definitely the way to go for a competitive arcanist.