Failed experiment with Google Webfonts

I've been implementing a new design for one of my websites, and my designer recommended using a couple of Google web fonts.

In the past, I've simply implemented custom fonts as images, which has the major disadvantage that changing the text requires creating a new image, and then tweaking the styling so that the new image fits the space available.  (It has the minor disadvantage of requiring a new HTTP request and the download time of the image, but if you use CSS Sprites, you can remove that extra HTTP request.)

So, I decided to try the new web fonts.  Google has largely solved the problem of acquiring the rights for new fonts, by requiring that any font in their library be licensed as open source.  They are also happy to host the fonts for you, so that they're always available and download quite quickly.  They even have beta support for a feature where you only download the images for the particular letters you're going to use -- if you only need 4 letters, why download an entire fontset?

Implementation is very simple -- the webfonts team has done an amazing job of making the process of selecting and implementing their fonts very intuitive.

But then I ran into a massive, deal-killing problem: Each user that visits your site will see a popup box (for each web font) that asks permission to download and install the font.  Ouch!  I can see why you might want that protection if you're installing a java applet or a Firefox plugin.  But isn't installing a font-set more analogous to downloading an image than installing code?

The popup includes text that looks something like this:

Firefox wants to use the font "Oswald Regular"
This font is not installed.  Allow Firefox to use this font?

Some subset of visitors (3%? 20%?) will see that warning and run screaming in the opposite direction.

So, I turn back to using images of words for this new design.

When might Google web fonts be acceptable?  IMHO: Only if you have a captive set of users using your webapp that you can train to "Allow" their browser to install the font.

Update:

I have gotten a couple of comments on this post, so I thought I'd provide an update.  I now think that this problem is limited to Mac users, and even then it happens only intermittently.  (You can see other complaints about this problem by doing the following Google Search.)

I actually decided to go ahead and use Google's webfonts on the site I was working on.  (Mac users make up a pretty small portion of my visitors (<2%) -- and they actually have a slightly lower bounce rate (about 1% lower) than Windows users.)  I take that to mean that when they do encounter this problem, it doesn't scare them away at too high a rate.