I currently use a Samsung GT-B2710 phone which has an inbuilt LED flashlight, Electronic Compass, FM Radio and GPS (which is sensitive enough to work indoors) using a JAVA app called 'UTM Data', which will convert the GPS WGS-84 coordinates generated from the internal GPS receiver to a host of other datums such as OSGB36, MGRS and UTM etc for paper map use i.e the UK Ordnance Survey 1:50,000 and 1:25,000 Topographic paper maps. The phone is unlocked, has excellent battery life, and easily charged using a Sunlinq 6W panel or external Lithium Battery pack etc.

www.youtube.com/watch?v=0vHfKq8CREk

Do not attempt this with your $600 iPhone.

For electronic mapping I also use a Ployer Momo 9 (Android) Tablet PC with the Maverick Pro App (no inbulit GPS or 3G, although a 3G USB dongle can be attached to the USB port). This allows me to cache most download maps that are available including OSGB (except it seems Google Maps). This gives me even more extensive electronic mapping capabilities for the UK than even the Satmap Active 10 Plus GPS (typically costs £300). SD cards for complete OSGB electronic map coverage of the UK may add another £300-400

The Ployer is cheap at around £100 and maps are cached on a 16GB micro SD card when working without any WiFi or 3G connection.

The main problem with electronic mapping and navigation is the compromise between display size and weight, battery life, fragility and the manufacturers insistence of using their own non standard mapping formats, for which you pay a hefty price, whilst the some companies (Google, Apple ahem) like to keep a track of where you are.

The main advantage is the amount of data that can be stored on a microSD is very impressive. i.e Coverage of just Scotland on highly detailed OSGB 1:25000 topographic paper maps would weigh many times the weight of the tablet computer itself.

If going lightweight for a weekend I would just take a paper map/compass and the Samsung Phone (mainly for communications if within network coverage and for the occassional GPS fix if visibility is very poor). The Ployer Momo 9 might come along if it was a bug out situation.