This is a difficult question to answer. In this economy, it goes without saying that the more business you have, the more attractive you will be to a new firm. After all, many firms are struggling, and the addition of a partner who brings in $4 million of business a year will only help the firm's bottom line. However, not all is lost if you don't have a multi-million dollar book of business. While more common in a flourishing economy, there are still firms with busy practice areas and a need for partners (with minimal books of business) to take the load off of the current attorneys in the group.
Moreover, there are firms that will take on partners with smaller books of business, especially if that partner has serious potential for growing this book and/or is willing to be compensated at a lower rate than he or she would otherwise be if they had a larger book of business. Believe it or not, there are many partners out there who ultimately decide to make a lateral move for reasons other than salary. Many of these individuals would gladly trade a percentage of their compensation for other benefits from a new firm (better culture, quality of life, more management opportunities, good quality people with which to work, etc.).
Make no mistake, portable business is very important—likely the most important factor for firms considering whether to hire a lateral partner, but there are opportunities out there for partners with smaller books of business. Consequently, do not assume that you are stuck at your current firm exclusively because you have less than $1 million worth of portable business.