Thanks again and sorry for the delay while I was choosing. I was asking a lot of people for their opinions, too. =)

I have decided to award 1st through 4th places to:

1st viruz $400
2nd miso $300
3rd MachineDesign $200
4th basz $100

I am very happy with the diversity of the work that everyone produced, and can't wait to pit the winning designs against each other in a real ad campaign and see how they all do.

Thank you everyone for your submissions. I am currently choosing the winners. I've decided that these are good enough that I'm going to award 4 places at $400, $300, $200, and $100 if that's cool with everyone.

I just found these additional limitations for using these ads on Google. Sorry for not knowing about these earlier:

Flash ads

* File Size: Flash ads must be 50K or smaller in file size.
* Animation Length: Animated ads are restricted to a maximum of 30 seconds (at a 15-20 fps frame rate), after which point they must remain static. These ads must also comply with the other animation policies.
* Flash Versions: All Flash ads must be published for Flash Player version 4-8.
* ActionScript Versions: Actionscript version 1 - 2 is acceptable.
* ClickTags: All Flash ads should support the clickTAG variable. The clickTAG is the tracking code assigned by Google to an individual ad. It allows Google to register where the ad was displayed when it was clicked and helps advertisers determine the effectiveness of their campaign.
o On any click, Flash ads should redirect to the URL specified in the clickTAG argument; there should be no other redirection in between.
o The variable name must be spelled 'clickTAG' (upper-case TAG; no space between click and TAG) and not 'click tag,' 'Click Tag,' or any other form.
o This is the proper code for the clickTAG parameter:

on (release) {
if (clickTAG.substr(0,5) == "http:") {
getURL(clickTAG, "_blank");
}
}

o Note that is not necessary to specify the destination URL for the ad anywhere in this code; this is taken care of through the usage of clickTAG. Also, depending on the structure of your Flash ad, it may be necessary to prepend "_root." or "_level0." to "clickTAG" above, resulting in "_root.clickTAG" or "_level0.clickTAG". It is strongly recommended to upload the ad into your account and verify that the ad is behaving normally prior to it going live, allowing time for any necessary changes.
* Tracking Event Constraints: AdWords does not allow the specific tracking of user interactions that don't involve clicks. Tracking parameters that are not allowed include:
o View-through tracking
o Key-modifier tracking
o Mouse location tracking
* Other Behaviors:The AdWords system reviews the code of all Flash ads as they are uploaded. The following coded behaviors have restrictions on their use:
o Extra Calls: Your ad code cannot make external server calls for additional Javascript or other functionality. All functionality must be localized to the code itself.
o Tracking Bypass: You cannot change the behavior of your ad to avoid or eliminate the AdWords redirect URL.
o Destination Changes: The ad should direct users to a web page with the appropriate destination URL. It should not open the destination URL within the ad IFrame itself.
o Integrated Audio: Your ad may contain user-initiated audio only.
o Cursor Behavior: Your ad may affect or change a user's mouse cursor (arrow) only after the user initiates an action.
o Loading Ad Content: Your ad may load images or another SWF file at runtime. However, your new content must adhere to the current player security model and incorporate the proper settings for your Flash ad to run in our system.
o Random Numbers: Your ad may not include code that generates or uses random numbers.
o Other Disruptive Behaviors: Ads may not be designed or coded to circumvent the basic and usual AdWords flow. This includes (but is not limited to) the ad being displayed normally to users, Google registering the impression, and a click directing the user to the destination URL indicated by the advertiser in their AdWords account.

I believe it is "Sui Generis"

An animated banner would be OK, but it would have to look better than the flash banner ads. =)

Max file size for Google ads at least is 50k. I'm not sure what the limit on other ad networks is, so I guess 50k.