I agree with Dave (above) - I don't think that a tablet computer would be rugged enough to serve as a GPS navigator on the tankbag.
Another concern is that power consumption of a tablet will be much higher than power consumption of a dedicated GPS. The power that a dedicated GPS would demand from your motorcycle (presuming you supplied it with 12 volts) is close to negligable - less than one amp (12 watts). Most netbooks and tablets require quite a bit of power, somewhere in the 60 to 100 watt range.
I would recommend you get a dedicated GPS for navigating, and a small netbook (not a tablet) for email, internet, etc. The advantage of a netbook over a tablet is that the netbooks are typically clamshell designs that close up to provide the screen with more protection than a tablet (with its exposed screen) would have.
Don't underestimate the mechanical stress that being carried on a motorcycle for long distances places on a computer. I used to carry an IBM ThinkPad in the pannier of my ST1100. After encountering a couple of problems with little internal parts coming loose, I switched to carrying a Toshiba Toughbook (a ruggedized computer designed for shop and outdoor work).
It doesn't sound like you need that level of computing power (I occasionally had to respond to requests from work while on tour), but the point about ruggedness still applies. If you get a small netbook, you can wrap it up in the middle of your clothing supplies, that will provide it with a reasonable amount of cushioning and protection while you are riding. On the other hand, putting a tablet on a tankbag will be (in my opinion) suicide for the tablet.