No, comcast cable is encrypted and therefore un-recordable. I did a test to verify this. I went to my grandmother who has comcast digital cable. (She's always been ahead of the curve) Anyway, I used a mythtv+ubuntu box with a haupuage 1250-mc digital tuner and tried to tune the digital. The result? Nothing. Only the free channels came through. I tried every variation of tuner settings I could think of like Qam-256, 128, etc. Broadcast freq table, cable freq table. Basically, none of those channels worked. What I found interesting was that her lcd tv was able to tune most of those channels without using a set top box, yet my digital tuner card could not. As of right now, I'm convinced that digital TV from comcast cannot be recorded by mythtv in a reasonable fashion. Please, if you personally have a tuner capable of tuning comcast digital cable, let me know as I am prepping the wiki page to say that digital tuners won't tune non-OTA channels.
According to this: http://www.mythtv.org/wiki/Hauppauge_HVR-1250
the 1250 support is digital only. I'm guessing that Comcast in your grandmother's area is still primarily analog. Her TV can tune analog or digital, hence can tune anything not encrypted. I'm really not sure what you mean by "free channels" though.
I have Comcast "digital" cable, and I'm able to tune most channels in the 2-99 range with my analog only tuner card (pvr-150). Comcast "digital" cable is not fully digital, that's the problem.
I'm quite certain that Myth/LMCE can capture any signal that a TV can pick up off cable without a cable card. The problem for me is channels are being converted from analog to digital and I know of no dual-tuner card that supports analog and digital in Myth. I could probably augment my current analog-only card with a digital only, then modify my schedules direct setup separating channels by digital/analog into two different line-ups, assigning them to tuner cars as appropriate. If Comcast is converting channels a few at a time that would be a huge hassle to try to keep up with so I'm not even going to try that route.
For another example of capturing tv from Comcast Digital cable, see tkmedia's post above (page 1), he's getting 20-30 channels with HDHomeRun (digital only).