On switching systems:
- K-r and K-5 have greatly improved autofocus over the K-7 and earlier generations, both continuous and low-light. That said, 7D is class leading in continuous, and it's unlikely either will come close to the 7D's AF.
- Since the K-x, Pentax really stepped up their ISO performance. K-r and K-5 look to be stellar. 7D/60D is a bit better than K-x (both being much better than the K-7 and K20D), but K-r and K-5 look to be significantly better than the K-x.
- Pentax does not seem to care about tethering, so I wouldn't expect it from them in the near future. If this is a requirement, switch.
- Local dealers probably won't be selling Pentax, so if this is the ease you are referring to, I don't think it will improve.
- You can, however, rent Pentax lenses from CameraLensRentals
- Price of lenses to me seems comparable, with some cheaper and some more expensive than Canon.
- Their DA* lenses and limited primes are similar in quality to the L-series glass and cheaper
- Pentax does not have specialty glass in production (super telephotos, etc)
- Cheaper third party lenses get the benefit of in-body IS on Pentax, making it a better bargain
- You can adapt Pentax lenses to Canon (albeit losing functionality)
- Pentax cameras, IMO, have the best ergonomics in their class and UI. I am not a fan of Canon's ergonomics and choice of front and rear dial, nor their menu system. Try them out in a store and see if it bothers you. Have not personally held the 7D.
To get an idea of what price you may fetch, check out pentaxforums.com. Unfortunately, many K20D owners are upgrading to the K-5, so it will be harder to get a good price for it. Prices range, I think, from $450 to $600. On the other hand, the 18-250mm is out of production, so it's fetching anywhere from $350 to $400+
As for 50D/60D/7D, depends on what you are looking for.
50D's only advantages on the 60D are build, lens-specific AF adjust, which is useful if you use lenses with very shallow DoF, and slightly faster fps.
60D/7D have a significantly improved sensor, among a bunch of other general things. While the 60D has a flip screen, the 7D is a serious camera with awesome AF performance, fps, build, WR, viewfinder, etc. What do you want to do with your camera?