The Google Maps API is probably one of the most popular APIs out there, and it’s not hard to understand why. There are countless applications to which mapping functionality can be applied.

For developers and businesses looking for powerful mapping functionality, the free Google Maps API has been a godsend. But earlier this year, Google announced that it would be implementing usage limits for the Maps API, and on Wednesday, it followed through.

Going forward, those using the Maps API will be limited to 25,000 map loads per day, or 2,500 map loads per day when the Styled Maps feature is applied. Anyone exceeding these limits can either reduce usage, pay fees for excess usage above the limits or purchase a Maps API Premier license.

The fees for excess usage range from $4 to $10 per 1,000 excess map loads. For instance, a user of the JavaScript Maps API v3 will pay $4 for every 1,000 map loads above 25,000., while a use of the same API with Styled Maps will pay $4 for every 1,000 map loads above 2,500 and $8 for every 1,000 map loads above 25,000.

For very popular sites,” Google notes, “Maps API Premier is likely to be a more cost effective option.” That makes sense, particularly for free, consumer-oriented websites that can’t stay under the limits. After all, generating more than $4 to $10 CPM in, say, ad revenue for every 1,000 pageviews that add to excess page loads may be a tall order.

The good news is that Google won’t be enforcing its new limits until 2012, but needless to say, some Maps API users probably won’t be happy with the changes in the first place.

And therein lies the rub: there’s no doubt that the Google Maps API offers a considerable amount of value, but unlike companies that launch their APIs on a paid basis, Google tends to find itself implementing revenue models after giving the farm away for free.

That could be smart, ‘bait and switch‘ is devious lock-in strategy if your users can’t easily turn to a competitor, but it also comes with risk.

Google’s AppEngine pricing changes didn’t go over too well, and while there’s a very good chance the limits and fees for the Maps API won’t cause a huge backlash because its position in the market is stronger, Google probably shouldn’t make a habit of launching new offerings without clear pricing models beforehand if it wants developers and businesses to trust it as a platform provider.