A ERJ-175 experienced a conflict with a helicopter after takeoff.

Narrative

Upon being issued our takeoff clearance from Runway 21; the Controller assigned us runway heading and referenced helicopter traffic south of the field at 1;500. The position we were in on the taxiway prevented us for scanning for the traffic. When we pulled out on the runway; I scanned for traffic and did not see the helicopter. I transferred control to the First Officer and he performed the takeoff. It was hot and we were relatively heavy so we had a long ground roll; rotating in the last 2;000 FT of the runway.As the First Officer rotated to the climb pitch attitude I looked up and saw a helicopter directly at our 12 o'clock position and I immediately felt our flight path put us in danger of collision with the helicopter; I instructed the First Officer to turn left. At this point; we were at approximately 100-200 FT and the gear was still down. As the First Officer began the left turn the Controller issued a new heading of 100 degrees. We did not receive a TA or an RA because I believe we began evasive action while the TCAS was still inhibited.Once we were clear of the helicopter; we finished cleaning up the airplane; and I believe we slowed our rate of turn to the left. We contacted departure who questioned our heading; we informed him of our evasive action and he issued new turn instructions to a 100 heading. The rest of the flight was completed without incident. I estimate our closest distance to the helicopter was approximately one mile horizontally and 500-800 FT vertically.The Controller never mentioned during the takeoff clearance that the helicopter would be crossing the departure path of the runway. Had he mentioned that; I would have not accepted the clearance given which was runway heading. The Controller created a serious safety issue by not delaying our departure to allow the helicopter to pass. Luckily the weather was VFR and we were able to perform evasive action to prevent a possible collision.