Yes...the BETA is available at least until the Oct 10th release, but Valve will be rolling out the new server builds like a day before the 10th, so on the 9th there won't be many BETA servers. Valves focus will switch to the retail release.
Currently there are around 350 servers and they are nearly all full...just after midnight of the 17th (last night) there were about 140 servers and they filled up in less than a minute. The TF2 BETA is currently the 2nd most popular multiplayer FPS (according to gamespy)...and while counter strike is way out in the lead by 6 times more players I should point out that many CS players aren't going to even try TF2 until release because many of them didn't like TFC and never played the original TF.
My prediction is that TF2 will break even with CS soon after release.