One of my group has been coming here regularly for several years. He reckons security has grown greatly in that time as a cause for concern.
One local expat (who has been here most of her life) was mugged and badly beaten last year, and now ventures out at night very rarely. Her home has been fortified.
Another - a more recent arrival - also avoids going out in the evening.
Our hotel (Mt Meru) gave friends a security guard to cross the street to a Chinese restaurant.
Not an attractive prospect.
This is definitely more of a warning than a danger, but expect very, very aggressive hawkers in Arusha. On our way to safari, we stopped briefly to leave some bags in the hotel we would be coming back to and check in on some plane tickets. Walking around town seemed like a good idea until we stepped out of the car. Immediately, we were surrounded by people trying to sell everything from souvenirs to pillows to sodas. We tried to walk a little bit, but were followed everywhere and finally decided to retreat to our vehicle. It was better in the car once we closed all the windows.
On our return trip, we also went for a walk. With a smaller group and a more purposeful direction, we were bothered much less.
We were told that Arusha is quite safe and there were no problems for solo Western travelers in the city itself. However, something could happen; there was a Belgium tourist held up in the downtown in broad daylight recently. So don't let your guard down. However, the probability of this happening is very small. The biggest threat most tourists will face is from getting too annoyed about the fact that Westerners can not walk two feet in Arusha without being approached by people trying to sell you stuff!!!