I have no idea.
I thought I read somewhere that Comcast planned to remove all analog everywhere within a couple of years, which makes sense as they could use the bandwidth, there are now more people who have digital TVs that can handle an unencrypted QAM signal (i.e. limited basic), and DTAs are cheap to hand out to users of older sets. There's no point in securing rights to 150 HD channels if you can only deliver 100 of them to your customers!
It looks like Philly is one of the first (if not the first) to go completely digital and Philly is definitely the first to get to 150 HD channels. My guess (and it is PURELY a guess - I am not an industry insider in any way, shape, or form) is that Comcast will remove the remaining analog in other markets over the next two years in a manner similar to how they rolled out the "World of More" across most of their footprint (they still haven't completed the job) over the span of a couple of years. Who knows when it will becomes Seattle's turn? Maybe there is a Comcast insider lurking somewhere that can share this info?