It's fairly challenging for a lawyer (or a company for that matter) to predict "when to start worrying about issues."  Every company that starts up is probably out of compliance with a slew of laws and regulations somewhere.  Every company should worry about securing .mobi (and other domain names) at some point.  Every company should have a document retention policy in place.  Every website should have a terms of use.  It's not easy in practice to determine when something is enough of an issue that you have to start throwing resources into it.  (On a sidenote, this is actually a key skill lawyers are called upon to provide, and it irks me always to see a lawyer provide advice that's totally out of proportion to the lifecycle or trajectory of a company.)  Things have obviously changed over the past 10 years with the internet, and with applications that can be quickly deployed and which are rapidly adopted. And there's also the whole "social media"/"viral thing". 

Twitter's case is fairly interesting from this standpoint.  The rate of adoption of Twitter has been pretty staggering. It's another story as to whether user's stick around and what Twitter does with them, but there's no denying that people have flocked to it in droves, in a short period of time. 

I previously flagged Twitter's privacy policy (which is simply drafted, but which would definitely cause issues in the context of an acquisition - nothing insurmountable, just the additional cost of notifying existing users and allowing them to opt out?).  Lately I've been watching Twitter's brand management, and I'm curious as to whether Twitter has any significant trademark/brand issues brewing.  They have a trademark application pending (or two?), but it seems like they've been fairly laid back about their brand management.  A few issues/stories I've noticed in the past few months:.

1. Third Party Use of the Twitter Mark:  Does it matter that Twitter freely allows people to incorporate the Twitter mark or variations of the mark in third party applications?  Examples abound, including "Tweetdeck," "twitterific," "twittonary," and "Twitpic".  Lists by Mashable are here and here. 
   
   
2. Third Party Use of the Twitter Logo:  Does it matter that Twitter freely allows people to incorporate the twitter bird icon (or something similar) in third party applications? 
   
   
3. The Fail Whale Logo:  Someone else actually designed Twitter's popular "fail whale" logo.  Can the person who designed it continue to use it or exploit it?  (See this NYT article.) 
   
   
4. The Bird Logo:  Same with the twitter bird:  "Twitter Paid $6 or Less for Crowdsourced Bride Graphic" (Wired).  This article notes that the "fail whale" was "purchased outright" by Twitter.  The bird is another story:

Simon Oxley, the Japan-based Brit who licensed the bird graphic to Twitter for the price of a sandwich, through iStockphoto, sits somewhere in the middle. ...  Unlike Twitter's "fail whale" graphic, which the company purchased outright after it appeared on iStockphoto, Oxley's bird design remains on the site where it can be licensed under the same terms Twitter received. Nothing is stopping people from making sites that ape, mock or build upon Twitter using its own official graphic.

I don't know the answers to what are probably fact-specific questions off-hand, but I suspect there are some interesting trademark issues lurking around here somewhere.  There's nothing stopping the guy who owns the bird graphic from slapping them on t-shirts and sweatshirts and selling them.  Or is there?  Does Twitter have a trademark usage policy or guidelines?

Also:  a friend asks via email whether it matters that Twitter makes its service available for free - i.e., whether Twitter satisfies the "use in commerce" requirement.  My recollection is that from a trademark standpoint, use in commerce does not refer to the narrow idea of sales - it encompasses distribution. 

Blackberry v. Twitter:  Evan Brown @ Internet Cases posted about Blackberry's possible opposition to Twitter's mark.  Blackberry didn't end up filing an opposition, and it looks like Twitter's mark should register in due course.  They have one trademark application for the word mark.  None for the design marks. 

NB:  here's a super-fun (some say scathing) five minute cartoon video about Twitter.