As someone already mentioned, this type of blooming is common when the sun is in the image and the D70's shutter speed is faster than 1/250th. I didn't always get the streak but I would often get a white blob instead of the sun that stayed white no matter how I post-processed the image. I was baffled and frustrated by this too until I read Thom Hogan's online review of the D70, in which he discusses this and other issues. I haven't used a D70s so I'm not sure about that. Just be conscious of your shutter speed and you should be fine.
I like how the D70 even comes with a shutter if it only actually uses it until 1/250th.. Seems like its only real purpose is to keep dust off the sensor!
Avoid shooting faster than 1/250 and never set your aperture wide open when placing the sun in your frame... I haven't faced blooming till now (after about 5000 shots), except the first days that I was testing how this thing appears - there have been quite a lot of discussions about it...
