I have a 51F59A and my previous HDTV was also a Hitachi CRT RPTV (43UWX10B). Neither set (nor any HDTV I believe) supports CC over HDMI. CC, if available, will work over component if it's a 480i signal only. 480p and HD signals will not support/don't carry traditional CC.
I also am a Comcast user. I have a Moto HD-DVR (6416 Series III). Both this HD box as well as it's predecessor that I used for about 2.5 yrs (6412) both have their own CC function that will work with ANY signal--incl. HD. Unfortunately, you have to turn the cable box off to access them. Imagine other Moto HD cable boxes work the same way.
Turn off the box itself (with the TV on, of course) and press MENU on your cable remote. This will bring up the Moto's operational menu screen. You should see settings for TV type (selectable btwn 4:3 and 16x9) and adjustable settings for both HD output (1080i/720p) and SD--480i/p (sent in 4:3 ratio, which activates your TV's gray pillarbox bars), Stretch (expands 4:3 SD to 16x9 aspect---UGLY!) and a bypass that will essentially upconvert all SD to whatever HD resolution it's set to. In your case, 1080i. Not really recommended due to the fact that this setting will have black 4:3 bars on ALL SD signals. If you watch a lot of SD cable, those black bars can pose a far greater burn-in threat.
OK...what about CC you ask? In the aforementioned menu, there are various settings for the Moto's CC functions. Ther first and most basic is the one that turns 'em on and off. There are several others you can play with that change the font style, size, color, background color (if any) of the captions. Can be set up to look more like the captions on a DVD than the traditional CC look. Kinda cool.
The fact that you have to turn off the cable box to access this is the only bummer. Currently isn't a way to switch them on/off without interrupting your TV watching. And if you have the DVR box, you can't access the menu while something is recording (can't turn off box w/o recording operation getting interrupted).
Although I haven't tried it--so I don't know that this works--but the F59s have the ability to do digital captions. You might want to try activating the Moto CC and then see if the F59 will turn these "digital captions" on and off. Worth a shot.
But, considering your current situation (hey, I've been there, too....and will be again in a couple of months), leaving the Moto box's own CC on for the time being may not be such a bad thing.
And if you don't have the HD-DVR yet, now--with a new little one around eating up your time whenever THEY feel like it
--is as good a time as any to upgrade to HD-DVR service. You can then watch your fave shows when the baby sleeps! Nice to watch what you want on your schedule.
As far as going w/ E* or D* satellite service, you'd be in basically the same boat in terms of CC. Although their monthly rates are less than Comcast, if you want an HDDVR, you will have to cough up about $200 up front for their "lease fee" and you might have to invest in an antenna for your HD locals. More likely that D* will carry (and compress the hell out of) your HD locals. Less likely that E* will carry them over satellite. Should be considerably less to switch to DVR with Comcast, and they generally carry most/all your HD locals and pass them w/o any additional compression. And neither satellite carrier offers OnDemand like Comcast. It's the main reason I've stuck with them, even after the last rate hike.
Good luck and congrats on the new little one.