The airport express is good because with an optical out you avoid grounding/electrical contamination and it has other features. However I do not know how well it provides audio during video playback, etc and if there are sync issues.
I test drove an iBook and really loved the quality trim, design, and software. Just got my rather loaded Dell 6000 and am having a miserable time trimming the dell crap from it (and trying to get my OS CD's shipped to me). The Dell is certainly a lot more noisy than the silent ibooks and the Dell's are considered rather quiet for PC laptops (although I'm still waiting to get my system up and running the way I'd like so I can try out some fan programs). Worse yet, I thought I'd be getting a much faster trackpad, but from what I see so far, its just as slow and doesn't have the 2 finger scroll grrr.
Once my wallet recovers I am thinking of getting a Mac Mini myself. I wouldn't have to worry about getting warranty like I would a laptop...the reliability of the Mac Mini is incredible and if you google'd Mac Mini failure you wouldn't find anything except what you'd expect in low harddrive failure rates.
Best thing is the last revision Mac Mini's come with faster but quiet seagates for hard drives. If I had to do it myself, I think those seagates would be on the short list for ideal hard drive for a quiet pc. They did a silent revision that they have yet to publicize because I am thinking that they are going to do a Mactel update soon to that model. However that doesn't make the last PPC G4 versions a bad deal keeping in mind you are still going with a rock solid reliable platform. Course maybe the new Mactel's may be rock solid as well, time will tell. Still when I consider that the G4's in Mac Mini's are CPU's that basically can run without active fans in an iBook, it makes me wonder if even any current or soon to be released Intel mobiles can run as cool. At best the new ones will have Yonah's, and at worse Celeron's (which still aren't bad since Celeron's are basically much like Centrino's but with worse speedstep or clock throttling for energy/heat).
FrontRow integration and other possible HTPC additions would be a reason to wait however. But it really doesn't matter since what you really want to wait for (and what I don't think could possibly be released in January at current pricepoint although I'd love it if it was), would be a Mac Mini that could easily decode h.264 streams through GPU/hardware acceleration.