Sponsorship from one's employer can be great, or can come back to kick you in the ass.
The main thing to keep seperate is your employee relationship, and your business relationship. Make sure to carry yourself in a professional business manner for the sponsorship relationship to work. To acheive this best, do all business/sponsorship relationship work after normal business hours. This way it doesn't appear to others like your'e doing sponsorship stuff on company time.
The main problem that I've run into is that other employees within the company will become jealous, thinking that youre' getting some kind of handout. (Remember, most of the general public don't understand the sales leverage a sponsorship can bring, and all the decisions that go into granting it.)
Then they will approach the same employer for sponsorship of their child's baseball team or Brownie's Cookie Sale and say, "Well, you did it for Joe Schmoe, why not me?" When this happens, the company will normally cease all inside sponsorships, because it's causing employee relationship problems. And the company, instead of intelligently providing all the facts to support your sponsorship and deny the other person's, will just quickly shut everything down.
Sponsorship from one's employer is usually a double edged sword.
But I have also found success in contacting former employers for sponsorship. That is only if you left employment with the company on good terms!