I'm in the same boat - heading to Vegas and looking at cellphone options. I've decided to go with Roam Mobility. They seem to have pretty good rates. You can buy their sim card for 19.95 (either via the internet with free shipping or at a local seller if there's one in your area). For around $16 you can get unlimited North American calling and texting and 400 MB of data for four days. $28 gets you 7 days unlimited text and calling plus 700 MB of data. They have 1, 3, 7, 14 and 30 day packages that you can mix and match to come up with any period of time you need (I'll probably buy a 3-day and a 7-day plan for the 10 days I'll be in Vegas). My son used them last month in Vegas and he was very pleased with their service. Finally, all costs are in Canadian $.

Edit: If you only want talk and text it's cheaper - 4 days unlimited would cost $12.