Many companies, especially larger organizations, don't want to admit to paying their employees to learn on the job. They want them to execute at peak level and learn after hours by getting approved certifications (certs are the mind killer). In my experience that's not the way learning happens. Only when you're neck deep in a project for a number of weeks (months) all the time do you really soak it in. Throughout this period you're continually motivated to make something work well and prioritize it over all else.
From my web wanderings I've read thousands of posts over the past year on startups. You could say I'm fanatic about startups that primarily leverage the Internet to grow. From this activity I've identified the most compelling reason to forge or join a nascent company is that they not only encourage you to take risks and learn, they expect you to. There's a tradeoff though. The required weekly hours to kick ass in a startup are somewhere between 70 and 18x7. If you're extra work life demands you work less, an extreme startup may not be your best option. There are plenty of more casual startup positions out there, but make sure both company and your own expectations are out in the open. If things aren't working out, expect to be looking for another position quickly. Match making in startups moves fast and in a few weeks on the job you and your employers/cofounders will know if there's a fit.
Hacker Founders/Early Employees
As a technical cofounder or employee, you can expect a steep learning curve when it comes web development. I find myself getting corrected by skilled hackers on a daily basis, while digging deeper into data flow through the web. I had no idea when I started exploring Rails late last year that I'd be spending so much time reading documentation on a slew of protocols, frameworks, and languages. If you're not hanging with the best open source hackers at the get go don't despair. Just a few years ago many of them were in your shoes. Their intimate technical knowledge and ability to communicate that understanding was hard earned (don't discount the latter skill).
Business Founders/Early Employees
You'll be learning just as fast, but with a different set of tools. Your responsibilities demand marketing acumen as you chart the startup's survival course through early customer funding or hooking outside investors. Landing outside funding from angels or VCs still appears like genie magic to me. Yet I recognize patterns in funded companies: they're lead by experienced entrepreneurs, they have proven early market traction, and they have substantial growth and revenue. Social proof is often referred to by early stage professional investors, but I'm not familiar with how first time founders earn that kind of trust.