Guelph, Ontario - 12 confirmed cases to date.

Peel Region, Ontario - 1 confirmed cases to date.
Toronto, Ontario - 9 confirmed cases to date. 

UPDATE - 5.12.2008 "Brampton, ON," (via CNW) - 1 new case, Peel Public Health reports its the 1st case of measles. The case has been linked the recent cluster of measles in downtown Toronto.

UPDATE - 5.09.2008 "Toronto's Financial District" Toronto Public Health authorities are warning of a measles outbreak Toronto's Financial District.  4 new cases (see below) have been reported in the downtown core, including at least two connected to the Scotia and TD Waterhouse towers. via: National Post.com

UPDATE - 5.09.2008 "Grey-Bruce area residents" It's possible some residents in the Grey-Bruce area have been exposed to measles. The Grey-Bruce Health Unit is "WARNING" the public of the situation.
Our Medical Officer of Health, Doctor Hazel Lynn, says a Toronto woman - known to be contagious with measles - took a bus from Kincardine to Toronto. Doctor Lynn says they'd like to know if anyone was on the bus, which left Kincardine on Tuesday, April 29th around 7:15 in the morning and we have a high immunization rate in Grey-Bruce area. Lynn says if symptoms don't show up by about May 19th - everyone should be in the clear. Via: Bayshore Broadcasting Corp.

UPDATE - 5.08.2008

Regarding Guelph cases: About seven of the twelve cases, where not immunized while a few other
cases where under-immunized, meaning only received one of the two shot
MMR vaccination  (which prevents the disease). Measles began spreading in Guelph last month after a local man caught the disease in Toronto and returned home.

Source re: Guelph via Guelph Murcury.com

Regarding Toronto cases: 4 new case (confirmed) reported 5.08.2008 Everyone affected by this newest outbreak of measles lives or works downtown.

Source re: Toronto via CityNews.ca & 680News

Symptoms:
Measles or Rubeola (rubella) is a highly contagious disease (spreads easily) through coughing and/or sneezing, along with close contact. Symptoms can include a high fever, coughing, sneezing, runny nose and rash. While the rash is the most visible of symptoms it is also one of the last and most noticed, the coughing and sneezing along with fever, these are the first signs that something is wrong. The fever is progressive or rises each day, and then the rash (blotchy) will appear around the face (head and upper body) and spreading to other parts of the body. It is also important to know that people with measles are infectious (can spread the disease) from 4 days before to 4 days after the rash appears. Thus, an infected person can spread the disease before knowing he or she is infected. Which why its important to be attentive of the fever, coughing and sneezing.

What to do - if you believe you have measles:
Persons with possible measles should wear a mask covering the nose and mouth, avoid public places, minimize contact with others, and notify their health care provider in advance to avoid patient waiting rooms. "Please do not just show up at a hospital, doctors office or medical facility without notify them first".

Why it's so important to be aware of measles:
CDC stated about how contagious measles was, "The thing about measles is that it's extremely contagious and that, if you put the measles virus in a room with unimmunized people, they would essentially all come down with measles. It's extremely, extremely contagious. 
It also persists in the environment for a couple of hours after the person has been there. And so we've even seen – in the older days, we saw times where, in the doctor's office, if the environment wasn't carefully cleaned up, somebody could get measles without ever having been in the office at the same time as the person – the first child."

How to prevent measles from spreading:
Get Vaccinated (Immunizes). Check with local and state health department for locations within your area for clinics and also possible free immunization site that are on going or being set up.