Solaris 2.6 and later (SunOS 5.6 -> 5.9) can handle files larger than this size as well. If you are using veritas filesystem you may need to make sure you are mounting with the largefiles option.
The only other problem you will want to avoid here is to make sure that you don't have any race conditions if you run the append on multiple threads or processes at the same time. You will want to look into locking in that case, or maybe a service that handles writing to the file and takes the input from a port or queue (think like syslog or sendmail).