For me, a senior developer is someone who is able to work in a TEAM, be dependable (i.e.: The guy who you can call at the 3am with an urgent matter), mentor others and be able to contribute and implement new processes and practices. The technical stuff is important but should not be the main filter IMO.
It also seems to me that you want to apply a certain logic or algorithm to the process. Personally I prefer to call up and just have a conversation with the person (trying to find out the aspects I listed above). Unless I am Google and I am getting a gazillion applications, that's different...
but, i still think that a longer-form, more essay-like quiz is better than grilling someone over the phone and expecting them to continuously talk to you while they're working through something.
Now I screen on much bigger topics that tend to be more philosophical in nature. The role of design patterns, benefits of one language over the other (particularly functional languages), value of unit tests...
They submit their answers via e-mail, and within a minute or two I already know if there is any value in at least talking to them in a more formal interview setting. I've found that folks who understand development at this much higher level always handle the basics well.
edit: i think i'd prefer having to do this myself, as well, because i'm a shy person in person and i find myself floundering when i'm on the phone and have to "explain my thought process" to someone as i'm working through it. i'd much rather write down an explanation.