I am going to have a project finished soon that will not have a usb/serial port on it. However i would like to have the option of connecting to the chip via its serial facilites for debugging purposes.
What i was thinking of was making a cable that a could attach to the serial ports of the AtMega when required. The cable would then be connected via some sort for conversion circuitry to my computers usb port where i would be able to read from/write to the arduino.
What you describe is my normal working practice. I use various Arduino clones (mostly the ModernDevice RBBB) with no USB circuitry on the Ardu-clone... it makes it much less expensive.
Described in more detail, including notes on where to buy the cable you mean, at....
I tend to "dedicate" the Arduino D0 and D1 pins to the needs of the development environment. If I want to do any other serial work, I "waste" two more pins, to keep life simple. To add a "serial port" using pins other than D0/D1, I use NewSoftSerial. Simple.
(Mega users already have three spare serial channels, don't need NewSoftSerial
I'm also a fan of Modern Device (I have 5 of their RBBBs). I use their serial level shifter for downloading, because my PCs have serial ports, but they also make a cheap USB-to-TTL-serial gadget.