Actually, I just got out of the Army not too long ago. I was a 91B (medical specialist). At the time, there was a separation between 91B and 91C (LPN). However, as of mid-February 2001, they were indeed merged to the mos 91W. 91C's were converted to 91W and obtained an identifier. The 91W school is 10 weeks long after basic training, and trains a person to be an EMT with minor bedside nusing skills. You would become a 91W10 upon graduation, until the rank of E-5 (sergeant) at which point you would become 91W20 (this is the designation for any MOS as you gain rank.), E-6 91W30, so on and so forth. I would NOT enter the Army now as a 91W, but I know friends that have entered and received an identifer. However, even if you have the identifier, it's no guarantee that you'll enter a unit where you would have a good experience to put your Lpn skills to work.
If you're looking to receive only LPN training, you may be better off looking into civilian schools. If you're looking for meaningful technical healthcare education in the military, I would advise looking into the Air Force. Not only do they have a healthcare tech program that is worthwhile, they have a better overall quality of life than the Army (my husband is Air Force). I've read (on these boards, actually) that after MOS graduation, you must complete two healthcare-related tests at your first duty assignment. This combined with (I think it's one year?) bedside patient care experience, allows you to challenge some state board exams for Lpn, such as California. I hope this helps, and best of luck to you.
As a side note, I continued with the Army reserves as a 91D, Surgical Technologist. In the military hospitals I've worked in, I've never heard of an Army LPN being allowed to first assist (though I have heard of them occasionally scrubbing in). Attaining first assist requires 1) Passing the CST (Certified Surg Tech) licensing exam, 2) Completing not only a certain amount of surgeries scrubbed in, but a high number of surgeries scrubbed in as first assist after you've passed the CST. 3) Passing the First Assist licensing exam. RN's are frequently sent through extra courses to become a scrub nurse, but even they require certification (and a great deal of knowledge on both ends about wound types/healing, sutures, A&P, surgical specialties, etc.)