What is the digital camera comparable for 28mm on regular 35 mm lens?

For point & shoots, look at the range, eg. 35-350mm. That's what you get. (The 3x, 5x, 12x zoom doesn't tell you where that range begins, so that's not a good indicator.)
For digital SLR cameras, the focal magnification factor is important. With all Nikons, that's 1.5. What that means, is that an 18mm lens will effectively work as 18mm x 1.5 = 27mm. With some Canons, you get a 1.6 factor, so an 18mm lens becomes 29mm.
Cheers!

There are very few cameras with such wide lenses. You can look at the Fuji Finepix S9000 (28-300) which is an excellent camera.

I take it you mean how many mm lens would be equivalent to 28mm on a 35mm film camera?
For canon D30/D60/10D/20D/30D/300D/350D it would be 17.5mm
For nikon D70/D70s/D200 it would be roughly 18.6mm

For compact digital cameras it really depends on the sensor size but they will normally report the focal length in terms of 35mm equivalent anyway.