Well, I have two responses to that.
1) While we have a good deal of control over 95% of the installer, we can't control everything. That includes the Agree/Disagree buttons! We may be able to hack a way around it, but I'm not sure its really worth it... See point 2:
2) There unfortunately isn't a good UI solution to this. The emphasized button (shown in darker blue) that responds to the return key should always be in the bottom right of the dialog. However, I agree with VISE not wanting to emphasize the Agree button, as it shows a sense of user initiative in agreeing to EULAs. While I don't want to get into a controversy over the use or need of EULAs (or their actual legal weight), users need not be agreeing to them completely second nature. Forcing them to find and click the agree button is at least the first step.
Now, what you are most likely referring to is that the OS X Installer has the Agree button in the bottom right, but gets around the HIG by not having it be emphasized. While I'm personally not a fan of this setup either, it technically breaks the rules (AFAIK) by not having *any* button be emphasized. Aple does have a pretty poor record of constantly changing and breaking their own HIG. [See the Print Dialog's PDF button/menu mishmash as an example] Unfortunately, VISE seems to be the correct one in this case, even if it means putting the agree button in a slightly nonstandard place. All that being said, I still have to defer back to point #1, that this is an issue best taken up with Mindvision, makers of VISE installers
if it is something you'd like changed for future installers.
Hope this helps, or at least doesn't cause too much dissention.