Update: I haven't worked on this in many moons and it has ceased to work. Here is a currently working MBTA map.
Following on the heels of Adrian Holovaty's work with Chicago Transit Authority maps, I dug around on the MBTA's website (Boston's T) for some decent maps. These are very clean and to-scale, so they were ripe for a Boston version.
After some fiddling around, I discovered that the Cambridge / Somerville inset appears to correspond with Google's tiles when the PDF is imported into Photoshop with a width of 1710px. After some slicing and dicing, I give you:
This is a screenshot of the Greasemonkey version (with minor alterations to Adrian's).
Building on top of some experimentation I'd done last week with Google Maps stand-alone, I made a non-Greasemonkey version by adding in a hook that effectively runs the Greasemonkey script. The default zoom-level is wrong, and my quick and dirty tile-serving code uses redirects (slowing things down and not handling missing tiles gracefully), so I'll post that later once I get a chance to clean it up.
Update: I don't have enough time to keep going on my own, so I've posted my work and a basic knowledge dump.
Update: Ok, so I found a bit more time and added
(the latter with the bigger map, thus covering the suburbs, though the North
Shore is a bit off). Adding additional zoom levels (
probably worthwhile to get more of an overview. Beyond that, what's next is
probably merging maps together to increase the coverage area and using the
insets for additional zoom levels.