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 to replicate this photo as closely as possible on your camera 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/2.0100mm 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. You can get very nice portraits with background blur with the 50mm but if you want to be able to do full length shots like the one you posted you need a longer lens, like the 100mm.