Is 720p vs 1080i worth being concerned about? Yes and no. If you're a consumer looking for a new TV, you can happily ignore the 720p vs 1080i debate because every TV which is described as HDTV or HDTV Ready is required to support both formats.
NOTE: You should be aware though that lots of TVs which support 1080i have fewer than 1080 lines and so scale the 1080 signal down. That's not a huge issue as even scaled down 1080i is far ahead of a regular NTSC signal. It is worth bearing in mind that more expensive HDTVs tend to have better scalers than cheaper ones, and this may be an issue.
However, for broadcasters it's a live issue. Should they broadcast 1080 lines of interlaced video or 720 lines of progressive scan? They could just broadcast two signals, one in each format, but that would use up a huge chunk of bandwidth and be hugely expensive for very little gain.
To answer the question, it's important to understand the difference between 720p vs 1080i. A 720p signal is made up of 720 horizontal lines. Each frame is displayed in its entirety on-screen for 1/30th of a second. This is know as progressive scan (hence the 'p')The quality is like watching 30 photographic images a second on TV. A 1080i signal comprises 1080 horizontal lines but all the lines are not displayed on-screen simultaneously. Instead, they are interlaced (hence the 'i'), ie every other lines is displayed for 1/60th of a second and then the alternate lines are displayed for 1/60th of a second. So, the frame rate is still 30 frames per second, but each frame is split into two fields, which your brain then puts together subconsciously.
Most of the time interlacing works fine, but for fast moving images, such as sports like baseball and hockey it can cause problems which manifest themselves as a 'stepping' effect on-screen. Progressive scan signals don't have this problem and so are better suited to sports.
ESPN puts it like this: 'Progressive scan technology produces better images for the fast moving orientation of sports television. Simply put, with 104 mph fastballs in baseball and 120 mph shots on goal in hockey, the line-by-line basis of progressive scan technology better captures the inherent fast action of sports. For ESPN, progressive scan technology makes perfect sense.'
Bottom line? For us, as consumers 720p vs 1080i is not a debate worth worrying about, so you can relax and focus on all the other criteria on your list when you buy your next HDTV.