Actually, it doesn't make sense that the effects would've been rendered at 720p at all, especially since there were stations airing it at 1080i (and, besides, 720p effects would've looked just slightly better than the 480i CGI done on the last couple of seasons of DS9 and Voyager), and 1080i broadcast results in a picture that is about equivalent to a Blu-Ray 720p image: that's why Warner is able to offer the first five seasons of Smallville in HD on broadcast and iTunes, but not Blu-Ray (although Season 5 is HD ready as demonstrated by its HD-DVD release), since the amount of compression (and why a number of stations broadcast in 720p instead of 1080i) results in a dramatically reduced picture in terms of quality. For Enterprise too have been done in HD, they would've needed to have done the effects in 1080p, so that they could ensure future-broadcasts and home video releases in formats that offered better compression. Right now, Blu-Ray contains higher quality playback of HD programming than broadcast or iTunes. Of course I never did think the CGI ships in Enterprise looked better than the actual models used for the earlier shows (or even the TOS-R CGI).
And CBS won't have all the Enterprise's on Blu-Ray by the time of the new movie...the Animated Trek was Remastered in HD for its DVD release, so there is still one Enterprise that won't be on Blu-Ray. Of course, it'll be nice when Voyager's on Blu.