Are there any downsides to starting a domain name with a digit?
Big downside: Having to spell it to people vocally it's very ambiguous. This goes for having a number anywhere in the URL.
So many times I've heard radio advertisements for websites where they have to spell it out:
To take advantage of this great offer, just go to travelwithkidz.com.au - that's kids with a Z - very annoying for everyone involved.
Take for example, 2example.com
Tech Support: Go to 2example.com - that's the number two, not the letters two, so two the digit example.com
Grandma: It's not working
Tech Support: can you read back to me what you've got?
Grandma (slowly): tee-doubleyou-oh-tee-hach-eee-dee-eye-gee-eye-tee-eee-ex-aye-mmm-pee-ell-eee-dee-oh-tee-see-oh-emm (twothedigitdotcom)
Well, the people at 37signals or 7dana might disagree with that. If it makes sense to use a number at the start of the domain, use it. If not ... don't. I'd surely hate to type out one-hundred-and-one-dalmations, when 101dalmations would do just fine :)
I think the key here is to avoid nonsensical domains altogether, i.e. something like '303wee' when in fact you are selling designer handbags.
I don't think its going to effect your ranking any more or any less than any other bad name that did not start with a number. A bad name is just a bad name regardless.
As of this moment, of the top 500 global sites according to Alexa internet, only 11 start with a digit. 2.2% only. So a digit-started domain name can get traffic.
Here's the list (please note: I have not looked at these sites to see if they are safe for work):
2ch.net4399.com58.com2345.com360.cn1stcngame.com39.net51job.com1133.cc7k7k.com1stwebgame.com
For some reason this question reminds me of E-Mail Addresses It Would Be Really Annoying to Give out over the Phone, which is very funny.
But seriously, any domain name can be successful. Look at woot.com and ebay.com and craigslist.org: ridiculous names; great, successful sites.