Reading the EXIF info in that image reveals it was shot using a Canon 5D with a 70-200mm f/2.8L lens at 182mm

The 5D is a full frame body which gives a wider field of view at the same focal length. It also gives a shallower depth of field than an APS-C body like your t1i *when matching the field of view*. So do replicate this photo exactly with respect to field of view and depth of field you would need a **113mm f/1.8** 

As Canon don't make a 113mm lens, you'll be able to get pretty darn close with the 100 f/2.0, which is a very nice lens, though may be beyond your budget. To get a similar look without spending as much you could get a 50 f/1.8. However you'll have to be a bit closer and wont get the same degree of background blur, which usually requires a longer lens.