Artytheurge, an all new Theurge prestige class!

[Hazzardevil]

This is true of all theurge PrCs I know of, fact is you're doing both at the same time. As it is now, you can get to effective level 15 in, let's say Wizard, by level 16, dropping 1 single paltry level of spells for the sake of also counting as a level 11 Artificer for infusions/day (but at level 15 CL).
I'm not sure your aware of what the Artificer Infusion list is composed of, it consists of buff spells, and not a lot else.
This is frankly ridiculous in my mind, you're talking about a near-gestalt build PrC. It's now so far off-balance that the scales have been lost completely.
Artificer is not Tier 1 because of the Infusing, it is Tier 1 because of the ability to make magic items, and these magic items are going to be benefiting the whole party, not just the artificer.
If the balancing were up to me, I'd be far more tempted to drop spell/infusion gain COMPLETELY at levels 1 & 10, since this class is looking a bit pokey just now. At least that way, our Wiz 5 / Art 1 / Artytheurge 10 would only count as a level 13 Wizard, level 9 Artificer (at CL 13), with a couple of really nice abilities to augment his buff potential.
And then any optimizer would never take the 10th level, so far your the only person who things this class is overpowred, if I get 3 more people, I'll consider it.
This is before I even get to the fact that a Cleric could come through this the same way, and with the buff/debuff potentials of that mix, these guys could break encounters.
[B]Cleric's are overpowered, I hear ya.[BI]
With that all out of the way (sorry about the rant), perhaps it's time to address wording issues!

Infusing the Arcana needs expanding. After it says;

you should add:
'with an infusing time equal to or less than the casting time of the spell. This requires no additional time, the infusion is used as part of the same action as casting the spell, upon the same target as the spell. If the spell could target more than 1 ally (yourself included) then the infusion applies only to the first person touched.'
That wording wouldn't work, since infusions generally have a minute casting time, I'll change it to a minute casting time or less, or if the spell has a longer casting of it's duration or shorter.
I think that covers most eventualities!

Artificer Abilities should probably drop this line:

It's a nice idea, but with the 'feat swap' caveat you threw in a couple of days ago, the above line becomes a completely moot point.
Good point.
Familiar Homunculus could be simplified by removing the second line and putting in it's place 'Refer to table on page 53 PHB for these bonuses.'
Also, this ability is only any use to arcane (and not even all of them) casters, any other class won't get the benefit. I doubt it's a problem, but I feel a bit sad
I'll add that it works with special mounts and animal companions as well, also, theres a feat in complete arcane called obtain familar, what it does should be self explanatory.
That's about it for now!

Comments with Italicization
Edit: the whole quote is italicalized for no reasom.

[Veklim]

And then any optimizer would never take the 10th level, so far your the only person who things this class is overpowred, if I get 3 more people, I'll consider it.

2 things!

1. Any optimizer wouldn't touch Artificer in the first place, not enough support and munchkin potential.

2. There aren't many ppl who have given an opinion either way in fairness dude, and if it was as simple as saying 'they're not proper tier 1, so it's fine' I will draw your attention to what would happen if you could sacrifice a level of wizard to essentially gain up to +6D6 sneak attack, evasion and uncanny dodge, which is a damned sight less gain than several levels of infusions (which are more than just buffs, they're near-free buff/power extensions due to the abilities in this class).

However you wanna look at it though, this is basically a gestalt PrC pretending to be a theurge if you keep it at +1 spells AND infusions at EVERY level. It's just too much. Show me a single other (reasonably balanced) PrC which does this over 10 levels, and I may actually concede the point...

[Volthawk]

2 things!

1. Any optimizer wouldn't touch Artificer in the first place, not enough support and munchkin potential.

...

You do know Artificer is one of the Big 5 of Tier 1 classes, ie one of the most powerful classes in the game, right? Their 'support' is every magic item published in every book, and every spell published in every book. Being able to make any magic item 2 levels earlier than anyone else (apart from scrolls and the like), getting every standard item creation feat for free, and being really good at UMD is that good. So yeah, that's wrong.

[Veklim]

Well that just pushes further to the point that this is a horrifically unbalanced class then!

[Hazzardevil]

Well that just pushes further to the point that this is a horrifically unbalanced class then!

The Infusions of Artificer are not the broken part! That's what they do in their spare time! My class is not unbalanced by advancing 2 classes casting at a slower rate. A level 1 Artificer/Wizard 5/Artytheurge 10 will lose in a duel with a wizard 16.